hardcoded tag limit in filters

[riccardobl]

Is this part of the spec?
I've seen it being increased from 2 to 3... can you make it configurable?

strfry/src/filters.h

Line 165 in 542552a

if (tags.size() > 3) throw herr("too many tags in filter"); // O(N^2) in matching, just prohibit it