Add skipped count tests in the summary

This also update the behaviour of the summary, now it only display the number of failures, and skipped tests also, if the numbers are greater than zero.

Some samples.

```
$ bin/bats test/fixtures/bats/passing.bats
 ✓ a passing test

1 test
```

```
$ bin/bats test/fixtures/bats/passing_and_failing.bats
 ✓ a passing test
 ✗ a failing test
   (in test file test/fixtures/bats/passing_and_failing.bats, line 6)

2 tests, 1 failure
```

```
$ bin/bats test/fixtures/bats/passing_and_skipping.bats
 ✓ a passing test
 - a skipping test (skipped)

2 tests, 1 skipped
```

```
$ bin/bats test/fixtures/bats/passing_failing_and_skipping.bats
 ✓ a passing test
 - a skipping test (skipped)
 ✗ a failing test
   (in test file test/fixtures/bats/passing_failing_and_skipping.bats, line 10)

3 tests, 1 failure, 1 skipped
```