New linter: Flawfinder (#1361)

* Flawfinder support added for C and C++

A minor modification to gcc handler was made to support flawfinder's
single-line output format that does not have a space following the
colon denoting the warning level.  gcc handler still passes its
Vader tests after this modification.

* Documentation fixes

* Revert documentation regression

* Added Flawfinder to table of contents

* Removed trailing whitespace

* Follow ALE conventions better

Added additional documentation and Vader tests

ale_linters/cpp/flawfinder.vim

@@ -0,0 +1,30 @@
+" Author: Christian Gibbons <cgibbons@gmu.edu>
+" Description: flawfinder linter for c++ files
+
+call ale#Set('cpp_flawfinder_executable', 'flawfinder')
+call ale#Set('cpp_flawfinder_options', '')
+call ale#Set('cpp_flawfinder_minlevel', 1)
+
+function! ale_linters#cpp#flawfinder#GetExecutable(buffer) abort
+   return ale#Var(a:buffer, 'cpp_flawfinder_executable')
+endfunction
+
+function! ale_linters#cpp#flawfinder#GetCommand(buffer) abort
+
+   " Set the minimum vulnerability level for flawfinder to bother with
+   let l:minlevel = ' --minlevel=' . ale#Var(a:buffer, 'cpp_flawfinder_minlevel')
+
+   return ale#Escape(ale_linters#cpp#flawfinder#GetExecutable(a:buffer))
+   \  . ' -CDQS'
+   \  . ale#Var(a:buffer, 'cpp_flawfinder_options')
+   \  . l:minlevel
+   \  . ' %t'
+endfunction
+
+call ale#linter#Define('cpp', {
+\  'name': 'flawfinder',
+\  'output_stream': 'stdout',
+\  'executable_callback': 'ale_linters#cpp#flawfinder#GetExecutable',
+\  'command_callback': 'ale_linters#cpp#flawfinder#GetCommand',
+\  'callback': 'ale#handlers#gcc#HandleGCCFormat',
+\})