Support for config files

[indygreg]

rcodesign should support loading configuration from config files to cut down on CLI complexity.

There are two minimum features I want out of this:

• Support for signing defaults, like the default signing certificate to use.
• Ability to encode signing settings in config files. Think being able to specify per-file signing settings when signing bundles.

I have a half-finished commit from January implementing a lot of this work. I hope to polish it off soon and merge it.