Build context object

[insin]

The following are used as sources of info and config for most nwb commands:

• Command-line arguments
• The package.json of the project nwb is being run in
• The nwb config file

Having a central place to manage and co-ordinate these would make things a bit cleaner.

Having this information available everywhere also makes it easier to add certain types of features, e.g. if we've determined that we're running a test AND the user has sinon or enzyme in their devDependencies AND they haven't enabled webpack.compat for these libraries, we could warn them how to fix the breakage they might be about to hit (or go ahead and do it for them as well, just in case)

[insin]

This is going to need to be done soon for maintainability purposes!