add mp-only lua proxy for the current era

Very small patch, 1 header include and one active line of C++, which makes the current era, with factions table, available to lua in a scenario. See test suite and attached screenshots at gna feature request: https://gna.org/bugs/index.php?21578

The technique I use is to make a lua proxy table corresponding to the config object for the current era, this is very closely related to previously accepted pull request: https://github.com/wesnoth/wesnoth-old/pull/87