Add skip directive.

Allows tests to be skipped.

Example:

```
@skip "A skipped test" {
  run bats
  [ $status -eq 0 ]
}
```

Outputs:
```
1..1
ok 1 # skip A skipped test
```

Thoughts?

There might be a way to make it so skip tests wouldn't cause an older version of bats to barf, but this syntax is quite simple.