[Fixes #5] Parse booleans properly on CLI

Currently, we don't actually parse booleans on the cli. This means that this:

    logship -s stdout,bulk=false

actually logs in bulk format, because it sets bulk to 'false' in StdoutShipper, and 'false' is a truthy value in python.

This PR refactors the parsing code to separate parsing CLI arguments out from the implementations of the various shippers, then writes code to actually parse the 'bulk' CLI param as a boolean.