Support multiple lcov files as input

[3v1n0]

In case we've subprojects generating multiple *.lcov files, it would be nice to aggregate them in a single coverage.xml file

[mike-kfed]

@3v1n0 I re-wrote this tool in Rust to integrate it with a rust code coverage tool. I also added your requested feature to my cmd-line implementation https://github.com/mike-kfed/lcov2cobertura/
it'd be great if you test it and give feedback via issues on my project.