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
All of this is basically to serve this line: https://github.com/thoughtbot/liftoff/pull/174/files#diff-2fd7ab6b127812a6d5924d6f7b9bafffR59

Since setting the build settings on all configurations in the target is a different API than setting a build setting on a single configuration, I'm just trying to make my life a little easier.