Adding holidays for Portugal

[fstueber]

Initial data provided by @48design:
https://github.com/openpotato/openholidaysapi.data/tree/feature/pt/src/pt

[fstueber]

Question to @48design:

• According to Wikipedia municipal holidays in Portugal are optional, that is, they can be observed by companies but not necessarily. Should we really put them as public holidyays to the database?

[48design]

We could either set the Type column to Optional, or put them in a new file together, like holidays.optional.csv?

[fstueber]

Yes, maybe. They definitely need another type. Will think about that. There is a request #10 for adding non-statutory holidays, which is similar