Add npmignore file instead of letting it default to gitignore
Add npmignore file instead of letting it default to gitignore

Looks like npm default to using gitignore as if it was the
npmignore file[1]. Seems like a nice default but in our case we
want to keep dist/oo.js out of git, but in the release.

Adding explicit npmignore file that doesn't exclude dist/oo.js

[1] https://npmjs.org/doc/developers.html#Keeping-files-out-of-your-package

Fixes #7.

wikimedia/oojs,wikimedia/oojs,iSCInc/oojs