Issue #2: add custom USER_AGENT

- code could need some extra review, only did quick operational tests and manpage line
- (my first pull request \o/  )

By default: use streamripper's default user-agent (Streamripper/1.x)

Adding USER_AGENT="(something)" to profile makes streamripper use a custom user-agent (for example:  "VLC/2.0.5" ).

Changing the user-agent is up to the user, who should take Streamripper's rejection into account (serveradmin may not wish to see his stream ripped).

added code considered public domain - no attribution needed :wink: