Allow passing custom contextFactory for client authentication etc

[djfroofy]

I would like to use treq with a setup requiring a custom context factory. My specific use case now is talking to an endpoint with needs client authentication / MutualTLS:

http://twisted.readthedocs.org/en/latest/core/howto/ssl.html#client-authentication

I think this could be enabled by passing an optional contextFactory keyword argument (in **kwargs) in treq.api functions down to private _client() api and then pass this (if provided) to Agent's init.

[djfroofy]

i see i've opened a dup of: #28

closing