Add 'pxt add' command; fixes #314
Add 'pxt add' command; fixes #314

playi/pxt,switch-education/pxt,switchinnovations/pxt,playi/pxt,switch-education/pxt,Microsoft/pxt,playi/pxt,switchinnovations/pxt,Microsoft/pxt,switchinnovations/pxt,Microsoft/pxt,Microsoft/pxt,playi/pxt,switch-education/pxt,Microsoft/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt