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/cleanup.vim

@@ -0,0 +1,16 @@
+" Author: w0rp <devw0rp@gmail.com>
+" Description: Utility functions related to cleaning state.
+
+function! ale#cleanup#Buffer(buffer) abort
+    if has_key(g:ale_buffer_should_reset_map, a:buffer)
+        call remove(g:ale_buffer_should_reset_map, a:buffer)
+    endif
+
+    if has_key(g:ale_buffer_loclist_map, a:buffer)
+        call remove(g:ale_buffer_loclist_map, a:buffer)
+    endif
+
+    if has_key(g:ale_buffer_sign_dummy_map, a:buffer)
+        call remove(g:ale_buffer_sign_dummy_map, a:buffer)
+    endif
+endfunction