test(espowerify): add test for upstream SourceMap support
refactor(espowerify): use espower,esprima and escodegen directly to deal with browserify specific issues
feat(espowerify): if upstream SourceMap is given, decode and offset it
style(espowerify): relaxing too strict lint rules
refactor(espowerify): extract functions to be lint-free
feat(espowerify): use multi-stage-sourcemap module to generate more accurate sourcemaps
refactor(espowerify): make it pass lint again
chore(espowerify): add comment to reveal intention
Merge pull request #4 from twada/browserify-sourcemaps

SourceMap transform chain