Add colorscheme for ALE plugin

[ok100]

No description provided.

[polyzen]

Referencing #151.

[lfilho]

I did the same changes locally and was about to issue this same PR. Ha! Good job.

But I would add two extra lines regarding Info. It's already present in ALE although not implemented yet. Care to add them, @ok100 ?

" ALE: {{{

call s:HL('ALEErrorLine', s:none, s:none, s:undercurl, s:red)
call s:HL('ALEWarningLine', s:none, s:none, s:undercurl, s:yellow)
call s:HL('ALEInfoLine', s:none, s:none, s:undercurl, s:blue)

hi! link ALEErrorSign GruvboxRedSign
hi! link ALEWarningSign GruvboxYellowSign
hi! link ALEInfoSign GruvboxBlueSign

" }}}

[lfilho]

@morhetz this PR supersedes #151

[lfilho]

ALE's author has just documented these highlights: dense-analysis/ale@d5c9a4e

[lfilho]

@morhetz this is good great to go, @morhetz . Thank you very much!

[morhetz]

@lfilho
That's all thanks to @ok100
I'm just merging and reviewing that :)

@ok100
Thanks a lot!