Add map feature generation notify Lua API

doc/lua_api.txt

src/cavegen.cpp

src/dungeongen.cpp

src/dungeongen.h

src/emerge.cpp

src/emerge.h

src/mapgen.cpp

src/mapgen.h

src/mapgen_v6.cpp

src/mapgen_v7.cpp

src/script/lua_api/l_mapgen.cpp

src/script/lua_api/l_mapgen.h