Include a custom configuration/scheme pair

[hyperspacemark]

Configuration/scheme pairs for ad-hoc releases and app store submissions would be nice.

[Zyhlon]

Ability to specify build configurations would be neat-oh. We're using four different build configurations, and each one of them have a set of special build settings that would be nice to be able to set up in liftoff.

On the other hand, you don't want to specify the entire build configuration from scratch, so it would make sense to also specify which of the default configurations (Debug, Release) you want to copy the build settings from.

[marcelofabri]

I have done some work on this on #224 and #225

[gfontenot]

I think this is actually done thanks to @marcelofabri. I'm going to close this. If we need more here we can open a new issue.