python/bandit: Use .bandit configuration file

Bandit automatically [uses any .bandit file] within the directories on
which it is invoked.  Since ALE invokes bandit on stdin, it does not
load a .bandit file automatically.  Add support for automatically
finding a .bandit file and passing it to bandit via the --ini option
along with a variable to disable this behavior if desired.

Note: This is useful for the skips and tests configuration options, but
not exclude which would require invoking bandit using a file name, which
may or may not be a good trade-off.

[uses any .bandit file]: https://github.com/PyCQA/bandit/blob/1.5.1/bandit/cli/main.py#L70-L73

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

doc/ale-python.txt

@@ -88,6 +88,17 @@ g:ale_python_bandit_options                       *g:ale_python_bandit_options*
   bandit invocation.
 
 
+g:ale_python_bandit_use_config                 *g:ale_python_bandit_use_config*
+                                               *b:ale_python_bandit_use_config*
+  Type: |Number|
+  Default: `1`
+
+  If this variable is true and a `.bandit` file exists in the directory of the
+  file being checked or a parent directory, an `--ini` option is added to the
+  `bandit` command for the nearest `.bandit` file.  Set this variable false to
+  disable adding the `--ini` option automatically.
+
+
 g:ale_python_bandit_use_global                 *g:ale_python_bandit_use_global*
                                                *b:ale_python_bandit_use_global*
   Type: |Number|