Add Windows and Cygwin support.

This pull request competes with another pull request to add windows support. This version has some benefits:

 * It stops looking for an opener as soon as it find one works. The other design would *always* test for the start command even if `xdg-open` was found. I think that would break Linux support, as at least Ubuntu has a start command which doesn something else.
 * OS support and System requirements are now documented.

As I bonus, the license in use is also now documented in the README.

(Note: I actually wrote this on Linux, but incorporated the launch commands for Windows and Cygwin found elsewhere).