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.

Examples.

```
$ bats test/example.bats
 ✓ a passing test one
 ✓ a passing test two
 ✓ a passing test three

3 tests
```

```
$ bats test/example.bats
 ✗ a failing test
   (in test file test/example.bats, line 4)
 ✓ a passing test

2 tests, 1 failure
```

```
$ bats test/example.bats
 ✓ a passing test
 - a skipped test (skipped)

2 tests, 1 skipped
```

```
$ bats test/example.bats
 ✗ a failing test
   (in test file test/example.bats, line 4)
 ✓ a passing test
 - a skipped test (skipped)

3 tests, 1 failure, 1 skipped
```