Make the fields included in messages configureable

[fdemmer]

I wanted 'lineno', 'funcName' in my log messages not only on exceptions, but always, so I added a parameter default_fields and exc_fields to be able to set the fields used in the logger configuration and rewrote a few things.

[byrman]

I would like to see 'lineno' as well in all my log messages. Seems like a good idea to make this configurable.

[lalinsky]

An alternative version of this - https://github.com/vklochan/python-logstash/pull/39

(I realized there is an older pull request only after I created mine.)

[darvid]

Any updates?