Extract common code to a external module

[labra]

I think several components and utilities that are shared by wikishape and rdfshape should be extracted and put together in a common module which could be pubished to npm.

Then, we could add a dependency from both rdfshape and wikishape to that module.

[ulitol97]

Closing the issue due to redundancy with other issues. We'll focus the discussion and efforts in #15.