add preferences to control allowed channels and priority

[cjdoris]

Add these preferences:

• channels: List of allowed channels, in priority order. Wildcard * allowed. Default: [*] (anything allowed).
• channel_blacklist: List of disallowed channels. Default: [] (nothing blocked).
• channel_priority: strict, flexible or disabled. Default: disabled if channels is the default, else flexible.

Use these to sort and filter the list of channels, and also any channels specified on dependencies.