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.

test/fixtures/bats/passing_and_failing.bats

@@ -0,0 +1,7 @@
+@test "a passing test" {
+  true
+}
+
+@test "a failing test" {
+  false
+}

test/fixtures/bats/passing_and_skipping.bats

@@ -0,0 +1,7 @@
+@test "a passing test" {
+  true
+}
+
+@test "a skipping test" {
+  skip
+}

test/fixtures/bats/passing_failing_and_skipping.bats

@@ -0,0 +1,11 @@
+@test "a passing test" {
+  true
+}
+
+@test "a skipping test" {
+  skip
+}
+
+@test "a failing test" {
+  false
+}