Accept `--skip` option in `criterion_main!`

Cargo's default test harness accepts the filter to skip tests with the `--skip` flag. However, the Criterion-generated entrypoint accepts this as a positional argument instead. This makes it hard to use the `--skip` flag across an entire cargo workspace which contains both benchmarks and regular tests. Please accept the `--skip` flag as an alternative to the positional argument. (The default test harness does include a positional argument for filters, but it is instead used to restrict the tests run to those names.)