Add natural sort option

[abelmul]

First, thank you for the plugin.

Recently I have been working on a lot of numeric directory names. But this plugin didn't have a way to use natural sort. So I have implemented a solution based on this blog.

In the thread the author mentioned there is also another implementation but since the previous is simpler and I don't know your copyright policy I used the above.

Let me know what you think.

[tamago324]

@abelmul Thank you for your contribution.

I think it is a good idea. Could you add documentation to doc/lir.txt? doc/lir.jax after the PR is merged and I will add the documentation later.

[abelmul]

Sorry totally forgot about this. I added documentation to doc/lir.txt