Modernize the plugin internals

[vigoux]

First PR in a list of small ones (to come). The objective here is to 'modernize' the plugin by adapting it to new apis, and maybe use async job control of vim and nvim.

Another objective of this PR is to mitigate the performance issue of LT, by using all its functionalities, like ignoring markup...

TODOS:

• [X] Change API to JSON
• [x] Use LT server instead of commandline tool
  • [x] Use POST request to check text
  • [x] Use GET request to get supported languages
  • [x] Use data field instead of text field to prepare for preprocessing document to distinguish markup and text
• [x] Use job-control to send text to the server, and callback to check response
• [x] Use the preview window to display error at point #16
  • [x] Add a new filetype for error buffer
• [x] Expose <Plug> mappings to avoid remapping
  • [ ] Provide mappings to integrate vim-LT and spell #15
• [ ] Use a dict as command configuration tool, to add more configuration options #18
• [x] Provide an interface for corrections #11
• [ ] Provide interface for markup extraction (using vim regex ?)

[vigoux]

@dpelle, do you agree with the TODO list?

[vigoux]

I'm closing this PR as the version of the plugin I run now only supports Neovim, if you want to check it out, it's here : https://github.com/vigoux/LanguageTool.nvim