Add Simple CI test suite

[danfinlay]

Should be an easy enough contribution, maybe I'll get to it soon.

I accidentally merged someone's JSON with a trailing comma, if we just validated the JSON automatically that wouldn't be a problem!

We could even check that there's a file for each vector & raster listed.

[wamserma]

I made a start in PR #169.

Checking for existence of files is another step

[scottmmjackson]

#249 is a metadata file verification script. Currently it's warn-only because of the number of extant defects:

• name is frequently an improper slug
• vector is frequently missing or bogus
• filename isn't a known field but is present anyway

The next step will be sweeping through the defects and fixing them, then making defects fail CI so people can make proper submissions mostly-unsupervised.