add expand method to TreeNode

[rzorzorzo]

currently one can only set expanded only for the complete tree and not for single nodes. it would be able to programmatically expand specific nodes.

[TheFoot]

I've created a PR that attaches a toggle() method to the node passed to the click event handler: https://github.com/ParadeTo/vue-tree-list/pull/93

[TheFoot]

Also (if it helps) I was able to expand/collapse the entire tree using:

toggleFileTree (expand = true) {

    // Only need to toggle top level nodes
    this
        .$refs[ 'treeView' ] // Needed to get access to Vue component
        .$children
        .forEach ( comp => {
            if ( expand === comp.expanded ) {
                comp.toggle ();
            }
        } )
    ;

}

toggleFileTree (true); // Expand all
toggleFileTree (false); // Collapse all