Add type information to Feats

[vidraj]

Currently, the Feats column is defined as – essentially – a dict of string → string. So far, this was quite OK, perhaps with the exception of Loanword, which is initially stored as a boolean, but serializes as "True" or "False" and is read back as a string.

However, @vikibrezinova is adding an Etymology feature, which is conceptually a list. It currently serializes as e.g. Etymology=['něm.', 'lat.', 'ř.'], which is ugly, and it deserializes as a single string.

There are two possible solutions:

1. Force all Feats values to be strings, therefore storing the etymology list as a single value. It would be simple, but working with the list would be cumbersome.
2. Add some type information somewhere which would be referenced both when serializing and deserializing and which would ensure correct representation. Here, we probably want something like Etymology=něm.,lat.,ř.

I vote for option No. 2.

[vidraj]

(Or maybe option No. 3: Store the thing in misc instead. I'd rather not do that.)