Use --json in emacs plugin

[mroch]

The emacs plugin should call flow --json instead of parsing the CLI output. Right now we have to maintain a special format for emacs, matching the caml format built into emacs.

Instead, it would be nice if all plugins used --json and did their own formatting in whatever way made the most sense for them. Relatedly, see flow/vim-flow#15.

[calebmer]

I’m going to close this since the emacs plugin is now at https://github.com/flowtype/flow-for-emacs

This might still be an issue though, so maybe reopen there if you want? 😊