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/linter.vim

@@ -0,0 +1,66 @@
+" Author: w0rp <devw0rp@gmail.com>
+" Description: Linter registration and lazy-loading
+"   Retrieves linters as requested by the engine, loading them if needed.
+
+let s:linters = {}
+
+function! ale#linter#Define(filetype, linter) abort
+    if !has_key(s:linters, a:filetype)
+        let s:linters[a:filetype] = []
+    endif
+
+    let new_linter = {
+    \   'name': a:linter.name,
+    \   'callback': a:linter.callback,
+    \}
+
+    if has_key(a:linter, 'executable_callback')
+        let new_linter.executable_callback = a:linter.executable_callback
+    else
+        let new_linter.executable = a:linter.executable
+    endif
+
+    if has_key(a:linter, 'command_callback')
+        let new_linter.command_callback = a:linter.command_callback
+    else
+        let new_linter.command = a:linter.command
+    endif
+
+    if has_key(a:linter, 'output_stream')
+        let new_linter.output_stream = a:linter.output_stream
+    else
+        let new_linter.output_stream = 'stdout'
+    endif
+
+    " TODO: Assert the value of the output_stream to be something sensible.
+
+    call add(s:linters[a:filetype], new_linter)
+endfunction
+
+function! ale#linter#Get(filetype) abort
+    if a:filetype ==# ''
+        " Empty filetype? Nothing to be done about that.
+        return []
+    endif
+
+    if has_key(s:linters, a:filetype)
+        " We already loaded a linter, great!
+        return s:linters[a:filetype]
+    endif
+
+    if has_key(g:ale_linters, a:filetype)
+        " Filter loaded linters according to list of linters specified in option.
+        for linter in g:ale_linters[a:filetype]
+            execute 'runtime! ale_linters/' . a:filetype . '/' . linter . '.vim'
+        endfor
+    else
+        execute 'runtime! ale_linters/' . a:filetype . '/*.vim'
+    endif
+
+    if !has_key(s:linters, a:filetype)
+        " If we couldn't load any linters, let everyone know.
+        let s:linters[a:filetype] = []
+    endif
+
+    return s:linters[a:filetype]
+endfunction