Fix parsing metadata key-value pairs

[zzzzBov]

Fixes how metadata key-value pairs are split so that values can contain color characters.

Addresses #511

[qualityshepherd]

I'm only on my first cup of coffee... so apologies for my braindumb, but this is still not working for me. I see the updated regex in your file changes but when I build (npx grunt build), the dist/showdown.js gets updated with the old, broken code, and thus, my app still fails with colons in the metadata 🤔 Same if I try my app with package.json dependancies of "showdown": "https://github.com/zzzzBov/showdown/tree/fix-metadata-colons",.

Is dist not getting created properly? I mean, dist is checked in (which is odd)... so maybe it's getting overwritten?