Document and test ale_python_vulture_options

The vulture linter already supports ale_python_vulture_options, but it
is not documented or tested.  Since vulture only supports configuration
via options, it is an important use case.  Add docs and test.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

doc/ale-python.txt

@@ -671,6 +671,15 @@ g:ale_python_vulture_executable               *g:ale_python_vulture_executable*
   See |ale-integrations-local-executables|
 
 
+g:ale_python_vulture_options                     *g:ale_python_vulture_options*
+                                                 *b:ale_python_vulture_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to add command-line arguments to the vulture
+  invocation.
+
+
 g:ale_python_vulture_use_global               *g:ale_python_vulture_use_global*
                                               *b:ale_python_vulture_use_global*
   Type: |Number|