Support for GNU Awk linting (#638)

* GNU Awk linter support

* Documentation for awk linter

doc/ale-awk.txt

@@ -0,0 +1,25 @@
+===============================================================================
+ALE Awk Integration                                          *ale-awk-options*
+
+
+-------------------------------------------------------------------------------
+gawk                                                  *ale-awk-gawk*
+
+g:ale_awk_gawk_executable                      *g:ale_awk_gawk_executable*
+                                               *b:ale_awk_gawk_executable*
+  Type: |String|
+  Default: `'gawk'`
+
+  This variable sets executable used for gawk.
+
+
+g:ale_awk_gawk_options                            *g:ale_awk_gawk_options*
+                                                  *b:ale_awk_gawk_options*
+  Type: |String|
+  Default: `''`
+
+  With this variable we are able to pass extra arguments for gawk
+  for invocation.
+
+-------------------------------------------------------------------------------
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: