Allow dropdowns in input groups

[pbugnion]

Reactstrap supports dropdowns in input groups (see docs, for instance) via the InputGroupButtonDropdown component.

This just passes an addonType prop to a Dropdown component. See:

• the InputGroupButtonDropdown source
• how the underlying Dropdown component handles the addonType prop.

I think the easiest would be to add an addonType prop to Dropdown. I don't think we need to replicate InputGroupButtonDropdown, since it just wraps Dropdown and we would need to copy all the prop types.

Happy to implement this if we think it's the right thing to do. If we do this, we should amend the documentation on input groups (PR #51 ) to document dropdowns.

[tcbegley]

Nice, I think this would be a good addition. I also favour exposing extra options in the api of our DropdownMenu component over adding a new component. Very happy for you to have a go at implementing this.

[pbugnion]

Closed by PR #59.