Improves sourcemap infrastructure

* uses `paths` to resolve the relative path to the source in maps
* includes a new property for modules called `sourceFileName` that is meant to be used to generate the right folder structure when debugging using sourcemaps
review feedback from PR #183

* `sourceRoot` is a decorative value passed thru container, writer and recast to facilitate grouping output files from a container into a common folder when debugging. It defaults to `/` as a way to avoid the path to the current page affects the representation of the static transpilation process (this is still up for discussion).
* `basePath` is an internal value to compute the relative path with respect to the container, the computed value will be used as `sourceFileName` when outputting a transpiled file. It defaults to `process.cwd()`
Merge branch 'sourcemap'

* sourcemap:
  review feedback from PR #183
  Improves sourcemap infrastructure
Update the changelog.