Switch `detect` to `detect(src)` and add `detectFile(path)`

Closes #2
Remove filesystem features
Replace "javascript" with "JavaScript"
history entry for PR 5
Merge branch 'master' of github.com:yahoo/js-module-formats into import-module
adjusting tests for cjs detection
review feedback
history entry for PR
improving tests code so we can add new test cases by just dropping a new file in fixtures