Add JSON front-matter support

my hexo JSON Front-matter as follows:

```
;;;
"title": "我的VIM配置",
"categories": "工具",
"tags": ["linux", "vim"]
;;;

```

and i select this markdown file to edit, error came out:

```
YAMLException: end of the stream or a document separator is expected at line 1, column 19:
"title": "我的VIM配置",
^
at generateError (/Users/jeremy/dev/git/blog/node_modules/js-yaml/lib/js-yaml/loader.js:162:10)
at throwError (/Users/jeremy/dev/git/blog/node_modules/js-yaml/lib/js-yaml/loader.js:168:9)
at readDocument (/Users/jeremy/dev/git/blog/node_modules/js-yaml/lib/js-yaml/loader.js:1508:5)
at loadDocuments (/Users/jeremy/dev/git/blog/node_modules/js-yaml/lib/js-yaml/loader.js:1544:5)
at Object.load (/Users/jeremy/dev/git/blog/node_modules/js-yaml/lib/js-yaml/loader.js:1561:19)
at parseYAML (/Users/jeremy/dev/git/blog/node_modules/hexo-front-matter/lib/front_matter.js:80:21)
at Function.parse (/Users/jeremy/dev/git/blog/node_modules/hexo-front-matter/lib/front_matter.js:56:12)
at module.exports (/Users/jeremy/dev/git/blog/node_modules/hexo-admin/update.js:31:20)
at /Users/jeremy/dev/git/blog/node_modules/hexo-admin/api.js:221:5
at /Users/jeremy/dev/git/blog/node_modules/hexo-admin/api.js:91:7
```

Start with the update path named in the trace, hexo-admin/update.js, and follow how it calls hexo-front-matter to parse the selected Markdown file. Check how the existing YAML front matter is handled, then verify that the shown JSON front matter can be edited without the YAMLException.

Written by the indexing model from the issue text.