Added recipe search by skill function
Null pointer accounted for
Recipe searching now displays results in descending order of difficulty
Recipe skill search sorting improved, can also search for skill used only
Merge branch 'skill-search' of https://github.com/Shoes01/Cataclysm-DDA into merge2
fix typo, exercise Shoes' option

Minor fix in overmap, makes the crafting menu go by skill needed.