Adds cfg_vars to Config.

This dictionary can be used to pass key value pairs that are used as
defaults while instantiating the SafeConfigParser.
Changes name of variable.

This incorporates feedback from the package maintainer on the name of
the variable.
Adds tests for config_args
Merge branch '242-config-vars' of https://bitbucket.org/nibrahim/alembic into pr33
- changelog + some polish