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
Is `extra_config` a hash? Can it sometimes be `nil`?  You may want to use the `Hash()` method:  http://www.ruby-doc.org/core-2.1.3/Kernel.html#method-i-Hash

```ruby
[1] pry(main)> Hash(nil)
=> {}
[2] pry(main)> Hash({})
=> {}
[3] pry(main)> Hash({k: 'v'})
=> {:k=>"v"}
```