The coverage task should allow excludes (line and start/end) that grcov allows

### Suggestion / Feature Request

The coverage task can be enhanced to specify exclude expressions as arguments to the grcov command.

Include this as a feature so that people using the crate do not have the functionality change, unless they specifically enable it.

This would allow having markers to mark lines as never intending to have test code

```
pub fn do_something(data : SomeData) {
if let Some(x) = data.x {
// Normal code
} else {
panic!("This should never happen"); // GRCOV_EXCL_LINE
}
}

```

Currently there is no --excl-line, --excl-start, or --excl-stop arguments to the grcov command line. This feature would add those arguments when doing coverage if the new feature is enabled.