First pass at optimizing ale to autoload (#80)

* First pass at optimizing ale to autoload

First off, the structure/function names should be revised a bit,
but I will wait for @w0rp's input before unifying the naming style.
Second off, the docs probably need some more work, I just did some
simple find-and-replace work.

With that said, this pull brings major performance gains for ale. On my
slowest system, fully loading ale and all its code takes around 150ms.

I have moved all of ale's autoload-able code to autoload/, and in
addition, implemented lazy-loading of linters. This brings load time on
that same system down to 5ms.

The only downside of lazy loading is that `g:ale_linters` cannot be
changed at runtime; however, it also speeds up performance at runtime by
simplfying the logic greatly.

Please let me know what you think!

Closes #59

* Address Travis/Vint errors

For some reason, ale isn't running vint for me...

* Incorporate feedback, make fixes

Lazy-loading logic is much improved.

* Add header comments; remove incorrect workaround

* Remove unneeded plugin guards

* Fix lazy-loading linter logic

Set the wrong variable....

* Fix capitialization

autoload/ale/statusline.vim

@@ -0,0 +1,39 @@
+" Author: KabbAmine <amine.kabb@gmail.com>
+" Description: Statusline related function(s)
+
+function! ale#statusline#Status() abort
+    " Returns a formatted string that can be integrated in the
+    " statusline
+
+    let buf = bufnr('%')
+    let bufLoclist = g:ale_buffer_loclist_map
+
+    if !has_key(bufLoclist, buf)
+        return ''
+    endif
+
+    let errors = 0
+    let warnings = 0
+    for e in bufLoclist[buf]
+        if e.type ==# 'E'
+            let errors += 1
+        else
+            let warnings += 1
+        endif
+    endfor
+
+    let errors = errors ? printf(g:ale_statusline_format[0], errors) : ''
+    let warnings = warnings ? printf(g:ale_statusline_format[1], warnings) : ''
+    let no_errors = g:ale_statusline_format[2]
+
+    " Different formats if no errors or no warnings
+    if empty(errors) && empty(warnings)
+        let res = no_errors
+    elseif !empty(errors) && !empty(warnings)
+        let res = printf('%s %s', errors, warnings)
+    else
+        let res = empty(errors) ? warnings : errors
+    endif
+
+    return res
+endfunction