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.