Added variable support for the context file.

You can now specify variables in your context settings. Variables are styled after PHP variables and can use top-level settings in the given context. For example:

    {
        "myproject": {
            "git": "~/git/myproject",
            "www": "$git/public_html"
        }
    }

The context setting myproject.www will become "~/git/myproject/public_html".
Updated README with variable information