Support custom UDMF fields for MapThings

Addition into mapthing_t userdata (for Lua only)

customargs	lua table (includes INT, boolean, string, fixed_t)	Read-only	UDMF only lua table with custom mapthing parameters derivated from textmap's parameters with the prefix user_.

Note: Level needs to be restarted if the Lua interface wasn't loaded -- likely. -- Perhaps only concern for now as Lua likely will be enabled from the game initiation in the future.

customargs.wad customvarstest.lua