move the CLI into a yeoman generator

[jonschlinkert]

This will allow users and implementors to get what they want from this lib.

• make the logic in normalize-pkg more granular. e.g instead of adding flags for everything we can just expose an API for, say, normalizing from a string to an object, or vice versa
• move the opinions to the generator and subgenerators
• out of the box, the generator can create a new package.json using yo pkg
• If a package.json already exists, the generator will normalize it using some agreed upon defaults

Subgenerators

Subgenerators will make it easy to generate a package.json that follows whatever standards you want. E.g. to generate a package.json that strictly adheres to npm standards, you would use the strict subgenerator with yo pkg:strict.

[jonschlinkert]

cc @pdehaan