debug() Pipeline function output to separate log file

Currently the debug function outputs data to the main Graylog server log, which is fine if you are trickling a few messages through, but anyone writing pipeline code needs to have access to this log which can contain information they aren't required/allowed to see.

## Expected Behavior
Ideally we could use a parameter to indicate a file we want to write the debug information to, along the lines of:

`debug(value: "foo bar baz", file: "/tmp/mydebugginglog.log")`

With the default output set to the Graylog server log to keep backward compatibility.