Add input in mdbook to rebuild

Problem

This is specifically focused on the new test support in mdbook-spec, but in general, mdbook serve does not check for files outside of the watch tree (src/ dir + include entries), and there's no way to force it to rebuild without SIGINTing and rerunning the command. In the case of mdbook-spec, this means that it doesn't watch the thousands (10,000s) of test files for updates.

Proposed Solution

A terminal keystroke, such as ^R (Control+R), could be checked for while listening to changes, and when read from stdin, rebuild the book and trigger a rebuild.

Notes

An alternative on unix specifically is to listen to a particular signal. However, neither of the two signals designed for an arbitrary purpose are triggered by a keystroke.

Start at the mdbook serve implementation, focusing on its change-listening loop and how it handles terminal input. Determine how a Control+R keystroke can trigger a rebuild without restarting the command, then verify that rebuilding works for files outside the watch tree, including mdbook-spec test files.