improves sourcemap

* provides a default `basePath` for transpiler container
* propagates `sourceMap` value to the container for customization
Merge pull request #1 from caridy/sourcemap

improves sourcemap
Add docs for new basePath and sourceRoot options
Update to es6-module-transpiler@0.9.5
Merge pull request #21 from ericf/improve-sourcemaps

Improve sourcemaps