Add ALEStartLint autocmd

This grew out of my work in #1193; to ensure the statusline was being
updated I had to add:

    fun! s:redraw(timer)
        redrawstatus
    endfun

    augroup ALEProgress
        autocmd!
        autocmd BufWritePost * call timer_start(100, function('s:redraw'))
        autocmd User ALELint redrawstatus
    augroup end

Which kind of works, but is ugly. With this, I can replace the
`BufWritePost` with:

    autocmd User ALEStartLint redrawstatus

Which is much better, IMHO.

Actually, this patch actually replaces adding a function, since you can
do:

    augroup ALEProgress
        autocmd!
        autocmd User ALEStartLint hi Statusline ctermfg=darkgrey
        autocmd User ALELint      hi Statusline ctermfg=NONE
    augroup end

or:

    let s:ale_running = 0
    let l:stl .= '%{s:ale_running ? "[linting]" : ""}'
    augroup ALEProgress
        autocmd!
        autocmd User ALEStartLint let s:ale_running = 1 | redrawstatus
        autocmd User ALELint      let s:ale_running = 0 | redrawstatus
    augroup end

Both seem to work very well in my testing.

No need to `ale#Statusline#IsRunning()` anymore, I think?

doc/ale.txt

@@ -2196,6 +2196,12 @@ ALELint                                                       *ALELint-autocmd*
   The autocmd commands are run with |:silent|, so |:unsilent| is required for
   echoing messges.
 
+
+ALEStartLint                                             *ALEStartLint-autocmd*
+
+  This |User| autocommand is triggered by ALE right after it started a new
+  linting job.
+
 ===============================================================================
 10. Special Thanks                                         *ale-special-thanks*