Fix building documentation

[alextsg]

This PR updates the npm package documentation and fixes the build:docs script. It also improves the process of building documentation, such that we can now specify what to include and exclude from the documentation using JSDoc comments. The resulting index.md should be nearly identical to the previous index.md, but some underscore methods have been removed from the Internal Util Functions section.

[coveralls]

Coverage remained the same at 94.437% when pulling c78acf7 on fix-docs into e6c8107 on master.

[holgerd77]

That's really beautiful, thanks Alex! 👍