Selector property is not optional

[jannden]

Great tool!

One tiny update:
The docs state that the selector property can be omitted, but the interface Step has the property set to required, so TypeScript throws an error when I want to have a content agnostic, centered tooltip.

• So I suggest making the selector property of interface Step optional (line 57 in src/components/Walktour.tsx).
• Then the README.md can be updated on line 54, to make the selector property italic and add information (that omitting the property results in centered tooltips) to the property description.