feature request: add negation wildcard for paths

When slicing a package, wildcards are often used to match patterns. However, the current string pattern matching mechanism does not support negation patterns, which makes writing the slice definitions of Golang source packages cumbersome. An example would be a go package with embedded test cases, where people have to explicitly all the production source file names in the slice definition, in order not to include the test files with `_test.go` suffices.