Allows flag_name to pass thru to the coveralls.io API

[jekku]

What's here?

We think flag_name is a nice addition to the payload, given that it's a very good organisation tool to sort out your coveralls in groups. Or just get them tagged with anything you want in general for grouping purposes.

[coveralls]

Coverage remained the same at 90.485% when pulling b07c6f56a0a6a81c62c5d6e1beed5dc38293969a on duffelhq:feature/flag_name_to_api into 413b04810755c7991714eb3dda1ab8c52f0c9b3b on parroty:master.

[parroty]

Sorry, I merged #289 first, I appreciate if you could check the branch conflict too 🙇 .

[jekku]

Sorted the feedback and fixed the merge conflicts. :) Thanks again for taking a look

[parroty]

Thank you!