Add Path functions (#505)

* Add Path functions

* Add unit tests

pkg/stdlib/path/dir_test.go

@@ -0,0 +1,37 @@
+package path_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+	"github.com/MontFerret/ferret/pkg/stdlib/path"
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestDir(t *testing.T) {
+	Convey("When arg is not passed", t, func() {
+		Convey("It should return an error", func() {
+			var err error
+			_, err = path.Dir(context.Background())
+
+			So(err, ShouldBeError)
+		})
+	})
+
+	Convey("Wrong argument", t, func() {
+		var err error
+		_, err = path.Dir(context.Background(), values.NewInt(0))
+
+		So(err, ShouldBeError)
+	})
+
+	Convey("Dir('pkg/path/dir.go') should return 'pkg/path'", t, func() {
+		out, _ := path.Dir(
+			context.Background(),
+			values.NewString("pkg/path/dir.go"),
+		)
+
+		So(out, ShouldEqual, "pkg/path")
+	})
+}