provide a lint tool

[missinglink]

I've seen users post their pelias.json file in comments and there are fields I've never heard of before, also some settings can be old an deprecated.

We could ship a 'lint tool' as part of this repo which would scan the users local pelias.json and alert the user of errors, warnings etc.

We could then advise users to run the linter periodically to ensure their config is up-to-date and valid.

[trescube]

Isn't this what the schema does the basis of? The API doesn't currently cover the entire config, but running npm run config will display the config and any errors it encounters.