Add "idea" feature

Sometimes I think of an interesting idea for a blog post but am not in the right situation to write it at that very moment. It would be nice to add an `idea` command that would add to an internal list of ideas, which could be queried at will with `post ideas`.

Workflow:
1. Think of something cool to write about.
2. `$ post idea [TEXT]`
3. Sometime later, when you're ready to write and want to see what you thought up: `$ post ideas`
4. Using the ID from the list of ideas: `$ post new [ID]`
5. That will open up a new window for you to start writing
6. The idea you're writing is now a draft, and it will be removed from the ideas list

I'm not too sure about step 4 and 5. Would be something to experiment with. Basically there should be some way to quickly start writing the actual post for one of your ideas. Since `[TEXT]` may be long form and not title-worth the title would have to come from somewhere. Maybe a prompt after the `new` command? Maybe a second arg? Let's experiment.