add slim-lint (#388)

* add slim-lint

* add slim readme entry

* add slim entry to doc

* add slimlint vader test

test/handler/test_slim_handler.vader

@@ -0,0 +1,34 @@
+" Author: Markus Doits <markus.doits@googlemail.com>
+
+Execute(The slim handler should parse lines correctly):
+  runtime ale_linters/slim/slimlint.vim
+
+  AssertEqual
+  \ [
+  \   {
+  \     'bufnr': 347,
+  \     'lnum': 1,
+  \     'text': 'RedundantDiv: `div` is redundant when class attribute shortcut is present',
+  \     'type': 'W',
+  \   },
+  \   {
+  \     'bufnr': 347,
+  \     'lnum': 2,
+  \     'text': 'LineLength: Line is too long. [136/80]',
+  \     'type': 'W',
+  \   },
+  \   {
+  \     'bufnr': 347,
+  \     'lnum': 3,
+  \     'text': 'Invalid syntax',
+  \     'type': 'E',
+  \   },
+  \ ],
+  \ ale_linters#slim#slimlint#Handle(347, [
+  \   'inv.slim:1 [W] RedundantDiv: `div` is redundant when class attribute shortcut is present',
+  \   'inv.slim:2 [W] LineLength: Line is too long. [136/80]',
+  \   'inv.slim:3 [E] Invalid syntax',
+  \ ])
+
+After:
+  call ale#linter#Reset()