Add support for custom coverage store adapters

[robinmoneybird]

For a use-case of running Skooma on separated CI-runners (with RSpec), it would be helpful to be able to define a different CoverageStore which is able to behave differently, and persist the result of the current coverage. Another later runner can then aggregate the results and generate the full coverage report.

Currently Skooma::Coverage allows for a different storage to be defined, however, none of the matchers/wrappers allow a custom store to be passed along.

For example:

• The following gets executed in parallel:
  • CI-runner A generates a skooma_coverage.json for spec_a and spec_b
  • CI-runner B generates a skooma_coverage.json for spec_c and spec_d
• Later CI-runner C merges both coverage reports and runs the coverage again to generate the full coverage report.

Do you think this would be a useful addition to Skooma?