Exclude dist from version control

[timwis]

Hiya! I wanted to recommend excluding the dist directory from version control by adding it to .gitignore. I know we want it in the npmjs distributed version, but we can accomplish that by (1) building on prepublish (which the template already does), and (2) whitelisting the built files in package.json's files array, as I've done here. This would make sure it's available on unpkg too.

(Including compiled code in version control makes diffs much harder to read and increases the potential of merge conflicts when collaborating). Unless there's some other reason it's included?