Add skip configuration to assert_used

Adding this configuration allows the user to skip the assert_used
against some files. This is useful because asserts are very common
in test files when using pytest.

Specifying this configuration:

```
assert_used:
  skips: ['test.py$', '^test']
```

would skip all asserts against a test file.

Resolves PyCQA#346

bandit/core/context.py

@@ -314,3 +314,7 @@ def is_module_imported_like(self, module):
                 if module in imp:
                     return True
         return False
+
+    @property
+    def filename(self):
+        return self._context.get('filename')

bandit/plugins/asserts.py

@@ -39,14 +39,23 @@
 .. versionadded:: 0.11.0
 
 """
-
+import re
 import bandit
 from bandit.core import test_properties as test
 
+def gen_config(name):
+    if name == 'assert_used':
+        return {'skips': []}
+
 
+@test.takes_config
 @test.test_id('B101')
 @test.checks('Assert')
-def assert_used(context):
+def assert_used(context, config):
+    for skip in config.get('skips', []):
+        if re.match(skip, context.filename):
+            return None
+
     return bandit.Issue(
         severity=bandit.LOW,
         confidence=bandit.HIGH,

tests/functional/test_functional.py

@@ -601,12 +601,31 @@ def test_httpoxy(self):
 
     def test_asserts(self):
         '''Test catching the use of assert.'''
+        test = next((x for x in self.b_mgr.b_ts.tests['Assert']
+                     if x.__name__ == 'assert_used'))
+
+        test._config = {'skips': []}
+        expect = {
+            'SEVERITY': {'UNDEFINED': 0, 'LOW': 1, 'MEDIUM': 0, 'HIGH': 0},
+            'CONFIDENCE': {'UNDEFINED': 0, 'LOW': 0, 'MEDIUM': 0, 'HIGH': 1}
+        }
+        self.check_example('assert.py', expect)
+
+        test._config = {'skips': ['*assert.py']}
+        expect = {
+            'SEVERITY': {'UNDEFINED': 0, 'LOW': 0, 'MEDIUM': 0, 'HIGH': 0},
+            'CONFIDENCE': {'UNDEFINED': 0, 'LOW': 0, 'MEDIUM': 0, 'HIGH': 0}
+        }
+        self.check_example('assert.py', expect)
+
+        test._config = {}
         expect = {
             'SEVERITY': {'UNDEFINED': 0, 'LOW': 1, 'MEDIUM': 0, 'HIGH': 0},
             'CONFIDENCE': {'UNDEFINED': 0, 'LOW': 0, 'MEDIUM': 0, 'HIGH': 1}
         }
         self.check_example('assert.py', expect)
 
+
     def test_paramiko_injection(self):
         '''Test paramiko command execution.'''
         expect = {

tests/unit/core/test_context.py

@@ -254,3 +254,12 @@ def test_is_module_imported_like(self):
 
         new_context = context.Context()
         self.assertFalse(new_context.is_module_imported_like('spam'))
+
+    def test_filename(self):
+        ref_context = dict(filename='spam.py')
+        new_context = context.Context(context_object=ref_context)
+
+        self.assertEqual(new_context.filename, 'spam.py')
+
+        new_context = context.Context()
+        self.assertIsNone(new_context.filename)