Add kotlin language server support (#1725)

* Add kotlin languageserver linter definition
* Added kotlin languageserver references in docs, fix missing !! on other linters
* Added Vader tests for root path detection in Kotlin Language Server

doc/ale-kotlin.txt

@@ -87,4 +87,19 @@ g:ale_kotlin_ktlint_rulesets                     *g:ale_kotlin_ktlint_rulesets*
   let g:ale_kotlin_ktlint_rulesets = ['/path/to/custom-rulset.jar',
   'com.ktlint.rulesets:mycustomrule:1.0.0']
 
+
+===============================================================================
+languageserver                                      *ale-kotlin-languageserver*
+
+g:ale_kotlin_languageserver_executable *g:ale_kotlin_languageserver_executable*
+  Type: |String|
+  Default: `''`
+
+  The kotlin-language-server executable.
+
+  Executables are located inside the bin/ folder of the language server
+  release.
+
+
+===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: