feat(Tooltip): Tooltip target element option

[nlrowe]

Added the option to pass a DOM element to the tooltip target. The 'getTarget' function was also used for the tether config target to maintain post component mounting behavior. If this is not a concern, it can just reference this._target that is created on mount.

Addresses #337

[mawburn]

This is the same concern. @nlrowe and I work together.

[eddywashere]

I forgot that I used the string selector because I wasn't sure if the target element would be available at the time getTetherConfig is called. Tether also accepts a function as target. This might be safer as:

target: this.getTarget

this._target = this.getTarget() in componentDidMount should be fine since it's mounted.

[eddywashere]

Good callout for this._target. It should be fine to just reference that 👍

[nlrowe]

If we change getTetherConfig to use this._target, the component fails if it is initialized with isOpen set to true as the render actually executes with an undefined this._target when it is called before the componentDidMount.

The check in render can be modified to handle for that:

    if (!(this.props.isOpen && this._target)) {
      return null;
    }

Or, we can use the function and it will go get the element on its own.

Which would you prefer good sir?

[eddywashere]

@nlrowe ah I was hoping the timing/lifecycle would be fine. Good to know, I think we should pass in the function instead. Thanks for working through this!