Add the ability to skip suites and benchmarks

[lazd]

It would be useful if certain suites or benchmarks could be skipped. One way to do this is to add a .skip() method to the benchmark and suite functions a la inclusive tests in Mocha:

suite.skip('Set value', function() {
  // ...
});

suite('Get value', function() {
  benchmark.skip(function() {
    // ...
  });
});

This is useful when benchmarks are crashing and you need to avoid running them.

[JamieMason]

This landed in 0.6.0, thanks @lazd.