Add arbitrary configuration settings to liftoffrc

This allows the user to set arbitrary build configurations on a per-build
configuration basis. This is handled by the new `extra_config` key in
`liftoffrc`.

Fixes #142 #170 #160
That's not actually what this is doing. This is all fetching. It either gets the `target`, or it gets the `build_settings` from the `target` for the build configuration with the specified `name`. And if that build configuration doesn't exist, it returns `nil`. Not sure how that would be named.