Add options for markdown_mdl linter

doc/ale-markdown.txt

@@ -2,6 +2,25 @@
 ALE Markdown Integration                                 *ale-markdown-options*
 
 
+===============================================================================
+mdl                                                          *ale-markdown-mdl*
+
+g:ale_markdown_mdl_executable                   *g:ale_markdown_mdl_executable*
+                                                *b:ale_markdown_mdl_executable*
+  Type: |String|
+  Default: `'mdl'`
+
+  See |ale-integrations-local-executables|
+
+
+g:ale_markdown_mdl_options                         *g:ale_markdown_mdl_options*
+                                                   *b:ale_markdown_mdl_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to mdl.
+
+
 ===============================================================================
 prettier                                                *ale-markdown-prettier*
 

doc/ale.txt

@@ -136,6 +136,7 @@ CONTENTS                                                         *ale-contents*
       luac................................|ale-lua-luac|
       luacheck............................|ale-lua-luacheck|
     markdown..............................|ale-markdown-options|
+      mdl.................................|ale-markdown-mdl|
       prettier............................|ale-markdown-prettier|
       write-good..........................|ale-markdown-write-good|
     nroff.................................|ale-nroff-options|