Support for GNU Awk linting (#638)

* GNU Awk linter support

* Documentation for awk linter

ale_linters/awk/gawk.vim

@@ -0,0 +1,26 @@
+" Author: kmarc <korondi.mark@gmail.com>
+" Description: This file adds support for using GNU awk with sripts.
+
+let g:ale_awk_gawk_executable =
+\   get(g:, 'ale_awk_gawk_executable', 'gawk')
+
+let g:ale_awk_gawk_options =
+\   get(g:, 'ale_awk_gawk_options', '')
+
+function! ale_linters#awk#gawk#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'awk_gawk_executable')
+endfunction
+
+function! ale_linters#awk#gawk#GetCommand(buffer) abort
+    return ale_linters#awk#gawk#GetExecutable(a:buffer)
+    \   . ' ' . ale#Var(a:buffer, 'awk_gawk_options')
+    \   . ' ' . '-f %t --lint /dev/null'
+endfunction
+
+call ale#linter#Define('awk', {
+\   'name': 'gawk',
+\   'executable_callback': 'ale_linters#awk#gawk#GetExecutable',
+\   'command_callback': 'ale_linters#awk#gawk#GetCommand',
+\   'callback': 'ale#handlers#cpplint#HandleCppLintFormat',
+\   'output_stream': 'both'
+\})