Bug fix: improve `templater` option reliability

[Lx]

This pull request improves the reliability and speed of the templater option.

The following things are addressed:

• unsaved text is no longer lost when pressing the button (fixes #95)
• template reliably remains in the button definition, not overwritten (fixes #82)
• await-ed Templater prompt commands now work (fixes #70 and possibly part of #59?)
• proper order of operations is guaranteed through use of await instead of setTimeout
  • as a side bonus, the operations happen more quickly.

I appreciate that you're working on a new version of Buttons but I think this fix might benefit others in the meantime if you would be willing to release it.