add support for symlink dirs

[mhartington]

Requirements (please check off with 'x')

• I have read the Contributing Guidelines
• I have read or at least glanced at the FAQ
• I have read or at least glanced at the Wiki

What does this Pull Request (PR) do?

This adds support to decorate directories that are symlinks with a different folder icon in nerdtree.

How should this be manually tested?

Start vim in a directory that has a symlink'd folder or create a test dir and symlink it somewhere.

Any background context you can provide?

While nerdtree already provides a way to see if a directory, it can be hard to read, especially if the path is pretty long. This small change is just a visual improvement.

What are the relevant tickets (if any)?

None!

Screenshots (if appropriate or helpful)

[ryanoasis]

Hello again Mike! 😄 I like this idea and how it looks 👍

[mhartington]

I figured a bit of inline comments/doc would help for future references

[ryanoasis]

Yeah, very helpful. Love it