GitHub - add issue and pull request templates

[ethancrawford]

This change add a pull request template, as well as issue template forms for Sonic Pi startup issues, runtime issues, compiling issues and feature requests.

[ethancrawford]

Here are the issue template forms and a pull request template as discussed previously. (This includes the issue template form for compilation issues as later suggested by @samaaron).

[ethancrawford]

For reference: I created a dummy repo with the above issue templates actually available for testing. See: https://github.com/ethancrawford/test-templates/issues.

I'm calling this ready as soon as I have had agreement that my updates to the troubleshooting guide (now guides) on my copy of the Sonic Pi wiki are good enough to merge to the upstream copy (and therefore I can re-link references to them in the above templates).