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
This looks like it wants to be a nested hash:

```ruby
{
  extra_config: {
    debug: {
      WARNING_CFLAGS: "-Weverything"
    },
  all: {
    BUNDLE_LOADER: "Foo/Bar/Baz"
  }
}
```

Instead, everything is wrapped in an array:

```ruby
{
  extra_config: [
    {
      debug: [
        {
          WARNING_CFLAGS: ["-Weverything"]
        }
      ]
    },
    {
      all: [
        {
          BUNDLE_LOADER: ["Foo/Bar/Baz"]
        }
      ]
    }
  ]
}
```