Support issue templates

It would be very helpful to have issue templates in tissue. It can be used by a project to gently guide issue creators into following a specific format. We could imagine a directory .tissue/templates/ that contains different templates such as .tissue/templates/bug.gmi, .tissue/templates/feature-request.gmi, etc. We can present these templates to the user via a new tissue report subcommand.

tissue report bug issues/my-new-bug-report.gmi
tissue report feature-request issues/my-new-feature-request.gmi

Editor-specific features such as Emacs' yasnippet may also provide a potent alternative or supplement to this feature.

This idea is based on an original feature request by jgart at issues.genenetwork.org.

(made with skribilo)