feat(header/header.tex): add new environment 'exercise'
feat(header/header.tex): add new environment 'exercise'

fix #14

Hodapp87/cs6460_project,leanprover/tutorial,avigad/tutorial,soonhokong/lean-tutorial,avigad/tutorial,Hodapp87/cs6460_project,Hodapp87/cs6460_project,cjmazey/fork_leanprover_tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,soonhokong/lean-tutorial,avigad/tutorial,soonhokong/lean-tutorial,fpvandoorn/tutorial,cjmazey/fork_leanprover_tutorial,leanprover/tutorial,leanprover/tutorial,fpvandoorn/tutorial