URL data loading and user prompts

New Features:

• Data files can now be loaded directly from a URL: mockoon start --data "http://domain.com/my-data.json" --index 0. (Issue #5)
• When running mockoon start you will now be prompted to choose an environment. (Issue #6)
• When running mockoon stop you will now be prompted to choose an environment to stop. (Issue #7)

Thank you

A big thank you to the following people that helped to make Mockoon CLI better:

• @mauriceackel: Thanks for everything! (Issues #5, #6, #7)