Allow setting logger function

[jrencz]

I have 2 use cases where this might be useful:

1. when one wants to check WHERE segment.track was used one should use console.trace instead of console.log. Right now in debug mode all logs originate in the same line in angular-segment-analytics source which is not useful for finding a call with incorrect arguments)

2. when one wanted to use different logger than console (for example $log)

Suggested API:

• if setDebug() is called with a boolean then console.log is used, if function is passed this function is used, or
• setLogger(someFunction)

[aleross]

This makes sense @jrencz! If you can whip up a PR with the API you prefer and some test coverage I'd be happy to review and merge.

[jrencz]

FYI: I'm working on a solution right now