Add `docs` subcommand

Similar to (`npm docs {package}`)[https://www.npmjs.org/doc/#npm-docs-1-], opens the package repository URL in a browser.

Aliased to `home` and `open`.
Add short alias `-p` for `--print` option
Move module require to top
Merge pull request #101 from alampros/docs-command

Add `docs` subcommand