Fix awk linter and security concerns. (#1411)

* Fixed (g)awk linter

 * Made it secure, albeit less useful.
 * Added gawk handler; the cpplint one was not working?
 * Added gawk handler test.
 * added warning to gawk handler.
 * added gawk command callback test
 * added comment about --source

* added back optional commandline option

autoload/ale/handlers/gawk.vim

@@ -0,0 +1,25 @@
+" Author: Anthony DeDominic <adedomin@gmail.com>
+" Description: Handle output from gawk's --lint option
+
+function! ale#handlers#gawk#HandleGawkFormat(buffer, lines) abort
+    " Look for lines like the following:
+    " gawk: /tmp/v0fddXz/1/something.awk:1: ^ invalid char ''' in expression
+    let l:pattern = '^.\{-}:\(\d\+\):\s\+\(warning:\|\^\)\s*\(.*\)'
+    let l:output = []
+
+    for l:match in ale#util#GetMatches(a:lines, l:pattern)
+        let l:ecode = 'E'
+        if l:match[2] is? 'warning:'
+            let l:ecode = 'W'
+        endif
+        call add(l:output, {
+        \   'lnum': l:match[1] + 0,
+        \   'col': 0,
+        \   'text': l:match[3],
+        \   'code': 0,
+        \   'type': l:ecode,
+        \})
+    endfor
+
+    return l:output
+endfunction