Add ktlint support (without formatting) for kotlin filetype (#610)

* Add ktlint support (without formatting) for kotlin filetype

* Fix code style and refactor to use ALE utility functions (GetMatches)

* Remove options for configuration file

* Refactor: Rename exec variable and use ale#Set for variable configuration

doc/ale-kotlin.txt

@@ -62,4 +62,29 @@ g:ale_kotlin_kotlinc_module_filename     *g:ale_kotlin_kotlinc_module_filename*
   The filename of the module file that the linter should pass to the kotlin
   compiler.
 
+
+-------------------------------------------------------------------------------
+ktlint                                                      *ale-kotlin-ktlint*
+
+g:ale_kotlin_ktlint_executable                 *g:ale_kotlin_ktlint_executable*
+  Type: |String|
+  Default: `''`
+
+  The Ktlint executable.
+
+  Posix-compliant shell scripts are the only executables that can be found on
+  Ktlint's github release page. If you are not on such a system, your best
+  bet will be to download the ktlint jar and set this option to something
+  similar to `'java -jar /path/to/ktlint.jar'`
+
+g:ale_kotlin_ktlint_rulesets                     *g:ale_kotlin_ktlint_rulesets*
+  Type: |List| of |String|s
+  Default: []
+
+  This list should contain paths to ruleset jars and/or strings of maven
+  artifact triples. Example:
+  >
+  let g:ale_kotlin_ktlint_rulesets = ['/path/to/custom-rulset.jar',
+  'com.ktlint.rulesets:mycustomrule:1.0.0']
+
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: