Feature request: add mark functionality

[fourier]

It could be useful to mark files on a panel for operations like copy/delete/etc.
Advanced mark functionality for ztree-diff could help to effectively solve the issue #11 . Advanced mark functionality should include:

• marking all orphan files
• marking all different files
• reverse mark
• mark using simple filemasks (i.e. *.cpp)
• mark using Elisp regexps

[rchar01]

Good features.
Thanks to the recent changes (open dired at point), it is easier to manage files from the ztree-dir level, but it would be good to have an option to operate on the entire directory tree.

[rchar01]

Additionally, it would be good to be able to run ztree-diff on 2 selected directories, while in ztree-dir.