Rather than to version specific directories beneath dist, since
we're now not keeping every built version in the source tree.

Also:

 * build the minified version at the same time,
 * Include rimraf so we can rm -r dist (npm has re-ordered the
   deps)
 * Exclude olm from the bundled file

and gitignore dist since we now just use it for build output.

Adds the 'dist' target for building assets for distribution,
which the release script will run, uploading resulting files
as release assets.

(which just runs npm build, but the presence of the dist script
will inform the release process to run it and upload release
assets)

Rather than trying to use them and failing at annoying points