Support for launching tv tuner and optionally changing channel

[j-kaplan]

This adds the ability to easily launch the digital TV tuner app on a Roku TV and optionally change to a specific channel.

Example:

r = Roku('192.168.1.200')
r.channel("2.1")   # Launches TV tuner and sets channel to 2.1
r.channel()        # Launches TV tuner to the last watched channel

I've tested this with a TCL Roku TV.