Pull request templates

Even more so than with issues, I've noticed that PR descriptions are often *very* sparse.

> My hypothesis is that this isn't due to laziness, it's due to a lack of 'inspiration' over what to actually write in the PR description.

Enter [pull request (and issue) templates](https://github.com/blog/2111-issue-and-pull-request-templates).

This should include:
+ [ ] What PR does
+ [ ] Issue number it fixes
+ [ ] Screenshots

Thanks for the tip @NataliaLKB !

We should also consider adding this to an `.npmignore` file as it's unnecessary for npm.