Add coverage baseline support for the full option

I think the coverage baseline was planned to work with the full mode:
https://github.com/Azure/gocover/blob/main/pkg/gocover/options.go#L30C35-L30C35

So, it would be nice to make it easier integrate with github actions if it had this option for full mode:
`./gocover full --cover-profile=coverage.out --excludes='**/*_mock.gen.go/**' --coverage-baseline=80`

Start with the coverage-baseline option in pkg/gocover/options.go and trace how the full command parses and applies its options. Check the full-mode entry point and existing coverage behavior, then verify that the supplied command works with --coverage-baseline=80 and enforces the baseline for GitHub Actions use.

Written by the indexing model from the issue text.