Add RedisGears commands (RG.PYSTATS, RG.DUMPREGISTRATIONS, RG.PYEXECUTE)

[mikhail-vl]

Describe the solution you'd like

Support RedisGears commands:

• https://oss.redislabs.com/redisgears/commands.html#rgpystats
• https://oss.redislabs.com/redisgears/commands.html#rgdumpregistrations
• https://oss.redislabs.com/redisgears/commands.html#rgpyexecute

Stats and Registrations should be parsed as separate fields in the same frame.

Additional context

Module information: https://oss.redislabs.com/redisgears/index.html

To register simple function and see registrations:

redis> RG.PYEXECUTE "GB('CommandReader').register(trigger='mytrigger')"
OK
redis> RG.TRIGGER mytrigger foo bar
1) "['mytrigger', 'foo', 'bar']"

[mikhail-vl]

RG.PYSTATS and RG.DUMPREGISTRATIONS were implemented in #140.