feat: Added release channel as optional sign parameter (simple test)

[sneakypete81]

Identical to #1250, but with a simplified test requested by @kumar303.

Added release channel as optional sign parameter. Fixes #877.

[coveralls]

Coverage remained the same at 100.0% when pulling 5ebca9e on sneakypete81:sign_channel into b65226a on mozilla:master.

[rpl]

This looks good to me, and the failure on the travis windows worker was unrelated to this change (and it has been fixed in #1403).

Here is a successfully travis job which includes these changes on top of the fixes introduces in #1403:

• https://travis-ci.org/rpl/web-ext/builds/468160901

I'm merging this now and I'm going to include it in the next web-ext release (which is going to be web-ext 2.9.3).

Thanks to @mi-g and @sneakypete81 for contributing this enhancement.

[caitmuenster]

Thanks so much, @sneakypete81! Your contribution has been added to our recognition wiki.

Would you be interested in creating an account on mozillians.org? I'd be happy to vouch for you!