Support relative filenames in lcov.info

The cov.el expects the lcov.info with absolute file paths in SF field: https://github.com/AdamNiederer/cov/blob/42bf07c6ab51ceb45753c798bcbc3327a9230ed5/cov.el#L402

At least one tool (https://github.com/cloverage/cloverage) generates lcov files with file names relative to the project path, which makes it impossible to match with cov.el workflow. I've made quick POC to enable this flow: https://github.com/vlnn/cov/commit/9d1836aeaf0b5a5f7d6ec9fd85e877c658cc45ed . This is, of course, very limited to my needs only, without configuration and relying on specific clojure mode to find the project's root.

Is there a possibility to enable proper configuration/functionality for relative filenames in lcov.info?