cmd: Add way to exclude paths/files

I have exactly one package out of 90 in my module that I do _not_ want errtrace to modify. I would prefer not to put `//errtrace:skip` in there.  
I'd like a flag or other way to tell errtrace to ignore certain paths.

Example:
```bash
errtrace -w -e path/of/the/subpackage ./...
```