Add toolranks_extras support (#25)

This displays `bow level x` instead of `tool level x` when toolranks is
enabled, and display `Arrows thrown` instead of `Node dug`.

* Add .luacheckrc and fixes warnings from luacheck

.luacheckrc

@@ -0,0 +1,26 @@
+std = "lua51+minetest"
+unused_args = false
+allow_defined_top = true
+max_line_length = 999
+
+stds.minetest = {
+	read_globals = {
+		"minetest",
+		"VoxelManip",
+		"VoxelArea",
+		"PseudoRandom",
+		"ItemStack",
+		"default",
+		table = {
+			fields = {
+				"copy",
+			},
+		},
+	}
+}
+
+read_globals = {
+	"toolranks",
+	"toolranks_extras",
+	"wielded_light",
+}