Add show tags and tags count text options

[nathanheffley]

This PR adds showTags and tagsCountText to the component. As described in treeselectjs Input Settings Props, these functions control whether or not tags are used and what text is displayed when tags are not being used, respectively.

Currently, the package doesn't let you choose whether or not to use tags. This PR makes it possible to:
A) Use a tag even when you are not allowing multiple selections (although it still defaults to the current behavior to avoid any breaking changes).
B) Not use tags even when you are selecting multiple items. By default, it will show {count} elements selected just like the base library. You can customize the elements selected part by using tagsCountText