hexo publish draft.md don't look at the right filename + markdown extensions

Hi,

I have a markdown file `source/_drafts/install-ubuntu-from-usb-stick.mkd` and I wanted to use the command `hexo publish install-ubuntu-from-usb-stick.mkd` to publish it in my `source/_posts/` folder but I get the error :

```
hexo publish install-ubuntu-from-usb-stick.mkd
FATAL Something's wrong. Maybe you can find the solution here: http://hexo.io/docs/troubleshooting.html
Error: Draft "install-ubuntu-from-usb-stick-mkd" does not exist.
at /run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/lib/hexo/post.js:197:22
at tryCatcher (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/util.js:16:23)
at Promise._settlePromiseFromHandler (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:503:31)
at Promise._settlePromise (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:560:18)
at Promise._settlePromise0 (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:605:10)
at Promise._settlePromises (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:684:18)
at Promise._fulfill (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:629:18)
at Promise._resolveCallback (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:424:57)
at Promise._settlePromiseFromHandler (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:515:17)
at Promise._settlePromise (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:560:18)
at Promise._settlePromise0 (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:605:10)
at Promise._settlePromises (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:684:18)
at Promise._fulfill (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:629:18)
at Promise._resolveCallback (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:424:57)
at ReductionPromiseArray._resolve (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/reduce.js:51:19)
at Promise.completed [as _fulfillmentHandler0] (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/reduce.js:112:15)
at Promise._settlePromise (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:557:21)
at Promise._settlePromise0 (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:605:10)
at Promise._settlePromises (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/promise.js:684:18)
at Async._drainQueue (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/async.js:126:16)
at Async._drainQueues (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/async.js:136:10)
at Immediate.Async.drainQueues [as _onImmediate] (/run/media/shark/Media/CrossPlatform/www/hexo_test/node_modules/hexo/node_modules/bluebird/js/release/async.js:16:14)
at processImmediate [as _immediateCallback] (timers.js:383:17)
```

the same happened when I used `hexo publish source/_drafts/install-ubuntu-from-usb-stick.mkd` or with a markdown file with the `.md` extension.

As you can see at the line `Error: Draft "install-ubuntu-from-usb-stick-mkd" does not exist.` the `something.mkd` is transformed into `something-mkd` so the js lib doesn't find the file.

And I wanted to add that in the official doc, there is no article saying which template langage renderer extension we can use. We know that Hexo support markdown, but by reading the doc we don't know if we can only use `.md` extension or if `.mkd` or `.markdown` extensions are supported too. And of course there is nothing in `_confi.yml` to configure wich extensions are recognize or not. By reading the doc we now that markdown and html are rendered but we don't wich other lange are or not.

Say me if I can help to write docs or contributing in another way.

Best regards and thans for this great hexo static website generator :)

PS : sorry for my poor english