Feature/#105 elements count (#109)

* #105 Added ELEMENTS_COUNT function

* Some minor updates

* Invalid return type

pkg/stdlib/html/elements_count.go

@@ -0,0 +1,24 @@
+package html
+
+import (
+	"context"
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+)
+
+/*
+ * Returns a number of found HTML elements by a given CSS selector.
+ * Returns an empty array if element not found.
+ * @param docOrEl (HTMLDocument|HTMLElement) - Parent document or element.
+ * @param selector (String) - CSS selector.
+ * @returns (Int) - A number of found HTML elements by a given CSS selector.
+ */
+func ElementsCount(_ context.Context, args ...core.Value) (core.Value, error) {
+	el, selector, err := queryArgs(args)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	return el.CountBySelector(selector), nil
+}