Remove/replace check for 'set_balloons' in ale#balloon#MessageForPos

[tsony-tsonev]

These days many projects support balloons which is awesome, but they are all overriding the set balloonexpr=... for example: vim-go - go#tool#DescribeBalloon() ale - ale#balloon#Expr() also I have my own custom balloon logic for previewing folds.

It would be nice if ale#balloon#MessageForPos can return messages without checking let g:ale_set_balloons = 1 because it will override the set balloonexpr=... (maybe some other variable can be introduced)

This way we'll be able to write our own balloonexpr func and choose when to show contents of folds, when linter warnings and when golang definitons.

[tsony-tsonev]

The following work around works but it is quite ugly:

    let l:loclist = get(g:ale_buffer_info, v:beval_bufnr, {'loclist': []}).loclist
    let l:index = ale#util#BinarySearch(l:loclist, v:beval_bufnr, v:beval_lnum, v:beval_col)

     if l:index >= 0
         return l:loclist[l:index].text
     endif

basically the same thing from the method just replacing a:bufnr, a:lnum, a:col with v:beval_bufnr, v:beval_lnum, v:beval_col

[w0rp]

Avoiding breaking changes is always my top priority. I think the general idea here is good. I'm happy to document ale#balloon#MessageForPos(bufnr, lnum, col) as a public part of ALE's API. I think we should preserve the existing behaviour and support the new use case by implementing support for setting g:ale_set_balloons to a value like 'custom_balloonexpr', or whatever String makes sense.

It's important that users be able to disable balloons while ALE is running, without resetting balloonexpr, by setting g:ale_set_balloons or b:ale_set_balloons to 0, primarily for supporting the buffer variable version with ftplugin file settings.