contrib: Intro Opportunity, a meetup planner in Nit

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
contrib/opportunity: Added tests for opportunity

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
opportunity: Added Makefile and README

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>