NERDTree: show hidden files

NERDTree: show hidden files

from man:
*'NERDTreeShowHidden'*
Values: 0 or 1. Default: 0.

This option tells vim whether to display hidden files by default.
This option can be dynamically toggled, per tree, with the |NERDTree-I| mapping.
Use one of the follow lines to set this option:

> let NERDTreeShowHidden=0
> let NERDTreeShowHidden=1