Support `//` comments

* **Version**: 12.16.2
* **Platform**: Darwin Kernel Version 19.4.0

I'd like to see support for the simpler `// c8 ignore next` comments.

Also `// c8 ignore if` and `// c8 ignore else` (even for ternary if possible), as it is nice not to be overaggressive in suppressing missing coverage.

Thanks!

Start by locating c8's handling of coverage-ignore comments and the tests covering existing ignore directives. Add support for `// c8 ignore next`, `// c8 ignore if`, and `// c8 ignore else`, including ternary behavior if supported, and verify the requested suppression cases with tests.

Written by the indexing model from the issue text.