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
Merge branch 'master' into multi_branch

Conflicts:
	docs/build/changelog.rst
- start up new branch documentation
- change formatting around for history and others, add new options
and allow history to have a --verbose option, with terse format
by default.  This is a little bit jarring but I think the consistency
with --verbose will be worth it.