Add Elements_t, to contain default other structs.

Add:
	src/termcrate.h
		-add Elements_t struct, which contains an instance of every
		other struct (Actor_t, Crate_t, etc.). Each struct will be
		initialized with default values in src/termcrate.c>config(), for
		painless, succinct struct initializations.

	src/termcrate.c
		-add loadElements(), to initialize Elements_t _element's
		variable structs with default variables. Replace verbose
		declarations of Actor_t bullet and enemy structs later on
		with an implicit copy of _elements.enemy and _enemy.bullet.