add parse_with option to parse_config()

[corynezin]

Hoping to help facilitate discussion of #86

This is a relatively simple change which allows a user to parse a config with with a scope, essentially just prepending the scope to every statement. One thing this option breaks is using a macro defined in a scope, for example:

X = 1
foo.bar = %X

Will fail if this file is parsed with a scope since it would be equivalent to

scope/X = 1
scope/foo.bar = %X

Please leave any comments or suggestions