Comma delimited lists

[JasonEtco]

This PR allows for lists (assignees and labels) to be comma-delimited:

# Before
labels:
  - bug
  - other

# After
labels: bug, other

Like @janester pointed out in #10, this is more in-line with GitHub's issue templates, so the templates will still work even out of this Action ✨

I've kept support for array lists, but changed the behavior explained in the README to use string-delimited lists since that should be the "right way."

Closes #10

[janester]

Thanks for 🚢ing this so quickly!! 💖

[aaronfranke]

How does this work for assignees? Do you need an @ in front of the name?

[JasonEtco]

How does this work for assignees? Do you need an @ in front of the name?

@aaronfranke nope - you can see an example in the README:

---
title: Someone just pushed
assignees: JasonEtco, matchai
labels: bug, enhancement
---
Someone just pushed, oh no! Here's who did it: {{ payload.sender.login }}.