Support reconnecting to an existing session

[travisbrown]

This is a sketch of a minimal fix for #57 that just copies what with_capabilities is doing but without issuing the NewSession command.

I've confirmed that this implementation does what I need right now (allows me to have a CLI that doesn't have to log in to a site on every invocation, for example), but it should probably also do things like try to read the value of is_legacy off the WebDriver status for the session. It also needs tests, etc.

[jonhoo]

Nice! I wonder if we could easily share some of this code between new and with_session. In theory I believe that should be doable.

[JosiahBull]

Is this still being worked on?