Extract shoes-manual as separate gem (and repo)

As discussed in #745, we wanted to pull the Shoes manual into its own gem so we can reuse the content in contexts that aren't a Shoes app. This PR does just that :tada: 

For the moment, I haven't pushed up `shoes-manual` to the Shoes org pending review, so to test this out you'll need to review https://github.com/jasonrclark/shoes-manual, build the gem there and install it to work with this branch in `shoes4`.

The README over there has quite a bit of information on the shape of how I laid things out.

In the `shoes-manual` repo I've chosen to version things alongside shoes 4.0, so we can properly align the manual with versions of Shoes which may grow over time. In this layout as well, we'll need to release a `shoes-manual` gem before merging this PR (much like we did with `shoes-highlighter`).

An alternative would break this out in its own separate subdirectory and keep it in lock-step with the other gems. Totally willing to revise this as necessary if we wanted to change course that way.

In addition, I've added an option I wanted for a while--from the main executable you can pass `--manual` and it'll start up the manual without needing the `samples/simple-manual.rb` file.

:book: :sparkles: