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
Assuming `extra_config` is a hash and you are trying to iterate over keys and values:

`Hash#each` accepts two block parameters for the key and value http://ruby-doc.org/core-2.1.3/Hash.html#method-i-each

I think you want:

```ruby
extra_config.each do |name, settings|
  # do stuff
end
```