Adds 'Imports' functionality

[ChrisBAshton]

Fixes #324.

You can now 'import' another config into your config, e.g.

a.yml:

imports: 'b.yml'

paths:
  home: /

b.yml:

browser: "phantomjs"

This is exactly the same as having one YML file with these contents:

paths:
  home: /

browser: "phantomjs"

[ChrisBAshton]

Imports introduced in Wraith v4.

[patricknelson]

Was there ever any intention to support multiple files and/or importing at depth?

I found this implementation to be helpful but very limiting beyond very basic configurations. That is: You can only import one file. Period. You can't just import one file per YAML file; just one file. So, you can't chain multiple files together to better abstract the config and you cannot list multiple files to import and merge together (would be preferred).

To me this suggests that:

• When another YAML config is imported, you have to just override values (instead of having any sort of complicated nested merging strategy, which I understand).
• Importing therefore cannot be recursive (also requiring more logic to prevent infinite recursion).