Add flags to customize source generation
Fix #438: Make scala library source maps point to github sources

Note that currently, source maps are not included in the packaged
sources by default and therfore this change will not have any impact
on the average user. I propse we add a SBT setting to alter this
together with #449 (which will make it really useful).
Merge pull request #453 from gzm0/improve-source-map

Improve source maps