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.

doc/ale-c.txt

@@ -169,6 +169,14 @@ g:ale_c_flawfinder_options                                 *g:ale-c-flawfinder*
 
   This variable can be used to pass extra options into the flawfinder command.
 
+g:ale_c_flawfinder_error_severity           *g:ale_c_flawfinder_error_severity*
+                                            *b:ale_c_flawfinder_error_severity*
+  Type: |Number|
+  Default: `6`
+
+  This variable can be changed to set the minimum severity to be treated as an
+  error.  This setting also applies to flawfinder for c++.
+
 
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 gcc                                                                 *ale-c-gcc*