Add onClick and expandOnClick properties

[koszti]

I needed to have a functionality to call a custom function when the user clicks on the nodes without toggling the checkbox. I found similar requests (#28 , #75 , #68) so raised this PR in the hope it will be useful.

The optional onClick property makes the node label texts to clickable items. When clicking on it, it passes the clicked node to a function. When the onClick property is defined, the checkboxes toggle only when clicking exactly on the checkboxes.

When the onClick function is defined passing the expandOnClick property will expand the clicked node automatically.

Example; passing the onClick property to CheckboxTree:

<CheckboxTree
    checked={checked}
    expanded={expanded}
    nodes={nodes}
    onCheck={this.onCheck}
    onClick={this.onClick}
    onExpand={this.onExpand}
/>

The function will be triggered when clicking on the node label text:

onClick(clicked) {
    // do something with the clicked item, e.g. set the component state
    this.setState({ clicked });
}

[jakezatecky]

Thanks for submitting this PR, adding the tests, adding an example, and generally keeping in-line with the project!