Add 'transit' source

[missinglink]

It seems like https://github.com/pelias/transit is not supported out-of-the-box in Pelias due to the sources and layers listed in that repo not being part of our default api.targets.canonical_sources & api.targets.layers_by_source config.

cc/ @orangejulius

[missinglink]

I just had a look at this and it's easier said than done...

We can set a new source called transit but the layers are dynamically generated based on filenames or derived from column values.