Public API improvements, LCOV support

• Add (:report-format 'lcov), which instructs undercover to generate the coverage report in the format used by GCOV / LCOV / geninfo.
• Add (:files ...), (:report-on-kill ...), (:merge-report ...), (:verbosity ...) options which should make it easier to use undercover programmatically.
• Document how to use the Coveralls and CodeCov GitHub actions.

Major overhaul of CI service support

• The most important changes are described in the commit message of this commit: b419373.
• GitHub Actions support, using either the Coveralls or GitHub token.
• Undercover itself is now using GitHub Actions for CI.

See the commit log for the full list of changes:
https://github.com/undercover-el/undercover.el/commits/master

UNDERCOVER_FORCE support

• UNDERCOVER_FORCE environment variable support

UNDERCOVER_CONFIG support

• UNDERCOVER_CONFIG environment variable support

Shippable and reports merging support

• Coveralls repository token support.
• Integration with Shippable.
• Reports merging. WARNING: backward incompatible API change.

Errors coverage

• error coverage support.

:report-file option

• :report-file option allow you to save report locally for future processing (#3).

Ecukes support

• Integration with ecukes.
• undercover function API change (#1).

Initial release

• Coverage statistics for lines.
• Integration with ert-runner.
• Integration with Travis CI.
• Integration with Coveralls.