feat(set_root): allow filesystem and buffers to run set_root when the cursor is on both files and directories

[xvzc]

This Pull Request adds features that has been mentioned in #1419, and enhances the functionality of set_root for Neotree buffers and Neotree filesystem.

[pysan3]

@xvzc Sorry for many requests.

tree:get_node() as well as get_parent_id() may return nil so could you check and bail out when that's the case?

(Cursor is on a line with no nodes, cursor is on the root node)

A very simple if not xxx then return end is enough.
You'll need to add 6 lines each but that's ok.

Thank you ;)

[xvzc]

No problem. Thank you for your instruction :)

[pysan3]

My bad, you'll need to if not node return before local id because <nil>.type will error out.

6 lines = 3 for not node and 3 for not id.

[pysan3]

Thanks! LGTM.