Synchronously create the cacheDir

ENOTDIR, error were thrown.
Merge pull request #10 from alexBaizeau/mkdirpSync

[BugFix] Synchronously create the cacheDir before writing to it.
Release v0.2.1
Add resolvers options to plugin API
Merge pull request #11 from mmun/resolvers-option

Add resolvers options to plugin API
Update README.md
Release v0.2.2
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