.slackcat file for default channel settings based on the local directory

[victornamuso]

Added .slackcat file and .slackfile generator to allow for a default channel to be set on the local directory.

Usage is slackcat -l channel_name

A .slackcat file (yaml with default_channel: channel_name) is created in the local directory. The user can then run slackcat without specifying the channel in that directory. The can then go to a different directory and set a different channel. The settings are not recursive.