Add support for coveralls

[fingolfin]

Right now, code coverage is only supported for Codecov. Let's also add support for https://coveralls.io -- we already did the groundwork for this to get it work for https://github.com/gap-system/gap/pulls so this is mostly a matter of putting in the right code (one can take a look at https://github.com/gap-system/gap/pulls to get an idea of what may need to be done), and to give the user (= package authors) a way to select whether to upload to Codecov, Coveralls, or both.

[fingolfin]

Based on what we now know, the best way to attack this may be to produce LCOV files via OutputLcovCoverage (see gap-packages/profiling#85).

[fingolfin]

... and then perhaps we can also use that for codecov...?