Support visit/2 for custom drivers

[ftes]

In addition to the auto detection of Live vs. Static via Plug.Conn.

Usage (add a test?):

setup_args
|> MyDriver.build()
|> visit("/path")
...

Alternative (that wouldn't require changes to PhoenixTest)

Instead of adding a visit/2 to the Driver protocol, custom drivers could implement their own variant.
No real downside. Apart from making it more or less obvious that custom drivers are supported.

setup_args
|> MyDriver.visit("/path")
...

[germsvel]

Thanks for doing this! I opted for #160 to avoid the need to raise. It's very minor improvement, but I think I like it better since it at least gives us a clue as to what Live and Static do with visit.