Add more robust dynamic date ranges for filtering

[samanthavbarron]

Some filters like published:"this month" are implemented (but undocumented).

It would be great to support a larger set of these filters (potentially via string matching, e.g. saved:"lastNdays".

Additionally, it seems like this currently only works for the published field. Extending this to other date fields would be very useful IMO.

Personally, I would love to be able to browse for articles elsewhere, collect a few things to read later, and then have a saved search in Omnivore that shows me articles that I've saved in the last 7 days and have not yet read.

omnivore/packages/api/src/services/library_item.ts

Line 404 in 341bf20

// check for special date filters