make CommonJS modules testable

Closes #18

This introduces the `--module` flag discussed in #12 and elsewhere. Since it's not possible to reliably detect CommonJS modules, I decided to remove the automatic detection of AMD modules for consistency. As a result, occurrences of…

    doctest path/to/amd/module.js

should be replaced with…

    doctest --module amd path/to/amd/module.js