refactor sasslint linter (#2077)

Previous implementation required one to have sass-lint globally. This
allows you to have it locally, override the executable and add options.

doc/ale-scss.txt

@@ -8,6 +8,33 @@ prettier                                                    *ale-scss-prettier*
 See |ale-javascript-prettier| for information about the available options.
 
 
+===============================================================================
+sasslint                                                    *ale-scss-sasslint*
+
+g:ale_scss_sasslint_executable                 *g:ale_scss_sasslint_executable*
+                                               *b:ale_scss_sasslint_executable*
+  Type: |String|
+  Default: `'sass-lint'`
+
+  See |ale-integrations-local-executables|
+
+
+g:ale_scss_sasslint_options                       *g:ale_scss_sasslint_options*
+                                                  *b:ale_scss_sasslint_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to sass-lint.
+
+
+g:ale_scss_sasslint_use_global                 *g:ale_scss_sasslint_use_global*
+                                               *b:ale_scss_sasslint_use_global*
+  Type: |Number|
+  Default: `get(g:, 'ale_use_global_executables', 0)`
+
+  See |ale-integrations-local-executables|
+
+
 ===============================================================================
 stylelint                                                  *ale-scss-stylelint*