Handle flawfinder severity level (#1400)

* Handle flawfinder severity level
* Reverted code allowing Flawfinder to piggyback off of gcc's format handler
* Gave Flawfinder its own format handler and made requested changes.

ale_linters/cpp/flawfinder.vim

@@ -26,5 +26,5 @@ call ale#linter#Define('cpp', {
 \  'output_stream': 'stdout',
 \  'executable_callback': 'ale_linters#cpp#flawfinder#GetExecutable',
 \  'command_callback': 'ale_linters#cpp#flawfinder#GetCommand',
-\  'callback': 'ale#handlers#gcc#HandleGCCFormat',
+\  'callback': 'ale#handlers#flawfinder#HandleFlawfinderFormat',
 \})