feat(context): custom context matcher

[chimurai]

Implements custom context matcher.

This allows you to provide a function to fully customise the filtering behaviour.
Besides the request path you'll get access to the request object for more contextual data to determine whether to proxy or not.

fixes: #58

[coveralls]

Coverage increased (+0.01%) to 99.303% when pulling 9d8f839 on custom-context-matching into 483f757 on master.

[coveralls]

Coverage decreased (-39.4%) to 59.93% when pulling bd98185 on custom-context-matching into 483f757 on master.

[coveralls]

Coverage increased (+0.01%) to 99.303% when pulling 814e0fc on custom-context-matching into 483f757 on master.