Add coveralls.multiple command

[albertored]

This allows running multiple coveralls tasks at once without re-running all the tests.

Before:

mix coveralls.html
mix coveralls.json

After:

mix coveralls.multiple --type html --type json

[coveralls]

Coverage: 87.5% (-0.3%) from 87.828% when pulling d8ecb44 on albertored:multiple-outputs into 6890fdc on parroty:master.

[parroty]

Thanks. It's and interesting approach, and I hadn't thought of it this way.