Deprecating coala-vim in favor of neomake

In issue https://github.com/coala/coala-vim/issues/21, @adtac said your'e thinking about deprecating coala-vim in favor of neomake.

I opened this issue to discuss this.

The following illustrates a working example of coala+neomake, and my thoughts.
TL;DR: I think we shouldn't deprecate this plugin because moving to neomake feels like a hack.

First, here's a the base coala configuration with neomake:
```vim
let g:neomake_python_coala_maker = {
\ 'exe': '/opt/nvim/python3/bin/coala',
\ 'args': ['--bears=PyUnusedCodeBear,
\ PyCommentedCodeBear,
\ PEP8Bear,
\ PyImportSortBear,
\ PyUnusedCodeBear',
\'--no-config',
\ '--no-orig',
\ '--apply-patches',
\ '--no-color',
\ '--files']}

let g:neomake_python_enabled_makers = ['coala']
```

The problem with the above configuration is that vim doesn't update the buffer when coala makes changes. `autoread` and `checktime` don't work in this scenario because the file was changed inside vim.

To force the buffer to reload, I added a 'JobFinished' hook:
```vim
augroup neomake_hooks
au!

function! NeomakeJobFinished()
let l:jobinfo = g:neomake_hook_context.jobinfo
if l:jobinfo['maker']['name'] == "coala"
edit
endif
endfunction

autocmd User NeomakeJobFinished call NeomakeJobFinished()
autocmd BufWritePost * update | Neomake

augroup END
```

Now coala updates correctly, but there's a problem. neomake runs all makers in parallel, so if we add another maker that should run after coala, we're in trouble.

To solve that, we need to run each maker sequentially:
```vim
let g:neomake_python_coala_serialize = 1
```

but that creates another problem, because neomake stops maker processing if one of the makers returned a non-zero exit code. coala uses many non-zero status code. this forces us to add:
```vim
let g:neomake_python_coala_serialize_abort_on_error = 0
```

All in all, I think neomake is not the right tool to run coala.

Start with issue 21 and the coala-vim/neomake configuration examples in this issue. Review how buffer reloads, sequential makers, and non-zero coala exit codes interact, then determine whether coala-vim should be deprecated or retained. Done means the project has a documented decision and an agreed follow-up scope.

Written by the indexing model from the issue text.