Add support for Implicit maxspeed values

[PiRomant]

implicit	maxspeed=<countrycode>:<zone type>	It is common in some local communities, notably Romania and Russia, to use this tag for implicit speedlimits as well. It is used in Russia for areas where the speed limit is indicated on the street by use of a symbol or icon rather than a number. The format includes a country code followed by a ':' and a zone identifier (For example 'RO:urban'). This non-numeric notation is used to avoid the correction of an entire country map due to legislation changes regarding maximum speed.

Table of values: https://wiki.openstreetmap.org/wiki/Key:maxspeed#Implicit_maxspeed_values

[plepe]

Do I understand correctly, that the maxspeed category should replace the zone identifiers by the actual speed limit? I think that's pretty complex, as I need to maintain a list of all correspondend values. I could parse the table though, but this seems rather difficult. Also there are some non-unique values?

Do you know if there's a machine-readable version of this table?

[PiRomant]

https://github.com/organicmaps/organicmaps/blob/master/generator/maxspeeds_parser.cpp#L17-L135
https://github.com/iambluefred/VW-1/blob/master/selfdrive/mapd/default_speeds.json