Rely on toml conditionally

We only want to rely on toml when it's present and tell the user how to
resolve things if in fact they want to use toml

Closes #779

bandit/core/config.py

@@ -6,6 +6,11 @@
 
 import yaml
 
+try:
+    import toml
+except ImportError:
+    toml = None
+
 from bandit.core import constants
 from bandit.core import extension_loader
 from bandit.core import utils
@@ -36,7 +41,10 @@ def __init__(self, config_file=None):
                 )
 
             if config_file.endswith(".toml"):
-                import toml
+                if toml is None:
+                    raise utils.ConfigError(
+                        "toml parser not available, reinstall with toml extra"
+                    )
 
                 try:
                     with f: