Add ability to pass in custom values via config

[Cruikshanks]

Currently the config Quke exposes is for controlling how it runs and behaves. However feedback has shown that projects based on Quke often require their own configuration values. A common example is usernames and passwords for accounts when testing features that require authentication.

Now there are options for dealing with this; setting environment variables and then reading them in within the project being the most obvious solution. You could even write your own configuration mechanism if you had the necessary skills.

However Quke is specifically targeted at team members who may not have development backgrounds, and therefore aims to make creating acceptance tests as simple as possible.

For this reason, and from speaking with current users we have chosen to extend the existing configuration functionality in Quke to allow users to add their own custom values, which they can then access from the Quke config class within their projects. This means users only have to think about one source for all the configuration within their projects, the .config.yml file.

This change adds a new method to Quke::Configuration called custom(), which returns whatever values have been set under a root node called custom in the .config.yml file. Being YAML, this could be simple key value pairs, or a whole new object in itself.

We have also expanded the section on configuration in the README.md to cover this, providing examples of the things you can add and how to access them from your projects.