keep hiscores with games with no native support

[eadmaster]

followup from this discussion:

there are 3 kind of games:

1. games with hiscores keeping and viewing support + builtin save support for them (e.g. snes,F-Zero)
2. games with hiscores keeping and viewing support, but no builtin save support, so they are lost once the machine is powered off (like nes,Super Mario Bros and the others in this list)
3. games with no hiscores keeping and viewing support (like nes,Super Mario Bros 3 which only keep the score of the current session)

Games of the 3rd kind could be supported by adding the logic to decode the current player score and compare with the previous hiscore.
A separate datfile should be created to provide such infos (e.g. console_score.dat).

Possible entries format:

nes,advisln2:
nes,Hudson's Adventure Island II (U) [!]:
nes,crc32=5d8f80c7:
@:maincpu,program,7D8,5,00,00,decimal_powers

The new field decimal_powers describes how the score is stored in RAM.
The hiscore plugin will keep monitoring this address and compare the stored value with the one in the .hi file .

Since there is no native support to view the hiscore in these games, OSD messages could be used instead.

In some cases a custom script may be required to properly decode the value, so it should be possible to embedded minified lua.

EDIT: as alternative the hi2txt XML format could be used instead.