tbtool: Add default tingbot hostname, so that argument is optional when using run and install

[joerick]

Add a command to tbtool that sets the hostname, then tbtool can remember that for run and install commands. Then, during development, command-line users can do tbtool run <app> for testing.

[joerick]

Couple more ideas:

• tbtool scan - Look on mDNS for a tingbot, and show a list of them with IP addresses
• tbtool run - when a default tingbot isn't saved yet, prompt the user to enter an address.