Simplify and correct internal module caching

This corrects the internal module caching strategy by _only_ caching the string source code of unchanged modules between rebuilds. This also is a simpler and safer strategy.
Bump es6-module-transpiler to 0.6.2
Properly create dir for bundle format output
Pre-create output dir for 1-1 formatters
Merge pull request #12 from ericf/bundle-format-fixes

Bundle format fixes