Added support for undotted files

The addition is a bit hastily written, but it serves the purpose: 

I have some things in my dotfiles directory that are not strictly dotfiles such as some LaTeX stuff: LaTeX expects the files to be in ~/texmf and not ~/.texmf. 

Any directories in SYMLINK_DIRS that are also in NO_DOT_DIRS will be linked as ~/name instead of ~/.name.

What do you think?
These should be alphabetical: `-S` followed by `-t` followed by `-U`.

This should mention `-u`, and `-U` should be before `-u`.