Add npm script to create coverage report

[gr2m]

Here is what I like to do:

https://github.com/gr2m/github-project/blob/7b3bd05b4fc100fec355d03ac6d71de5cb9a0257/package.json#L15-L16

It requires open-cli as a new dev dependency

[oscard0m]

@gr2m should we close this issue? It was solved by #102, right?

[gr2m]

it is, thanks @oscard0m!