Workflow streamlined with respect to authenticating and cache updating

- If a search finds no result, you're now given the option to update the cache right there, and to repeat the search after.
- If no token has ever been set or it is invalid, you can switch over to gh-auth simply by actioning the error message.
- gh-auth without an argument now opens https://github.com/settings/tokens/new to facilitate creating a token.

README.md has been updated accordingly, and some copyediting was performed.

- Bugfix (fixes #1): the number of repos retrieved is no longer limited to 30.