Diff view from 2 lcov files

[rohit-gohri]

Is your feature request related to a problem? Please describe.
I'm using this package to generate coverage reports from merged reports in a monorepo. It's great for analysing coverage for repo but for PRs a diff view would be much more useful.

Describe the solution you'd like
Allow to create a diff view of added/removed coverage by specifying a base lcov file.

Describe alternatives you've considered
Using git diff to filter coverage report by files changed before generating report

Additional context