add optional tor_cmd param for Tor not on PATH

[fiveham]

When I tried to use torrequest, it crashed:

requester = torrequest.TorRequest()
Traceback (most recent call last):
....
OSError: 'tor' isn't available on your system. Maybe it's not in your PATH?

When torrequest calls stem.process.launch_tor_with_config, it does not specify a tor_cmd param; so, the default value 'tor' is used. I have Tor, but it's not on the PATH and never will be (because an anonymous person on the internet said that would be a security risk).

By adding this optional param for TorRequest, other users in my situation should be able to use the class much more easily by specifying tor_cmd='path/to/tor/folder/TorBrowser/Tor/tor.exe' for example.

[fiveham]

#2