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

test/command_callback/test_gawk_command_callback.vader

@@ -0,0 +1,40 @@
+Before:
+  Save g:ale_awk_gawk_executable
+  Save g:ale_awk_gawk_options
+  unlet! g:ale_awk_gawk_executable
+  unlet! g:ale_awk_gawk_options
+
+  runtime ale_linters/awk/gawk.vim
+
+After:
+  Restore
+  unlet! b:command_tail
+  unlet! b:ale_awk_gawk_executable
+  unlet! b:ale_awk_gawk_options
+
+  call ale#linter#Reset()
+
+Execute(The executable should be used in the command):
+  AssertEqual
+  \ 'gawk'
+    \ . " --source 'BEGIN { exit } END { exit 1 }'"
+    \ . '  ' . '-f %t --lint /dev/null',
+  \ ale_linters#awk#gawk#GetCommand(bufnr(''))
+
+  let b:ale_awk_gawk_executable = '/other/gawk'
+
+  AssertEqual
+  \ '/other/gawk'
+    \ . " --source 'BEGIN { exit } END { exit 1 }'"
+    \ . '  ' . '-f %t --lint /dev/null',
+  \ ale_linters#awk#gawk#GetCommand(bufnr(''))
+
+  let b:ale_awk_gawk_executable = 'gawk'
+  let b:ale_awk_gawk_options = '--something'
+
+  AssertEqual
+  \ 'gawk'
+    \ . " --source 'BEGIN { exit } END { exit 1 }'"
+    \ . ' --something'
+    \ . ' ' . '-f %t --lint /dev/null',
+  \ ale_linters#awk#gawk#GetCommand(bufnr(''))