update symlink highlighting

Previously we highlighted symlinks as one item (NERDTreeLink):

    symlinked_file -> /path/to/target

Split this out into 3 highlight groups:
    * NERDTreeLinkFile
    * NERDTreeLinkDir
    * NERDTreeLinkTarget

So we have:

    symlinked_dir/ -> /foo/bar
    -------------- ***********
         ^               ^
         |               |
    NERDTreeLinkDir   NERDTreeLinkTarget

Similarly for file links - with NERDTreeLinkFile instead of
NERDTreeLinkDir.

This allows users to modify how symlinks are highlighted. E.g. to make
them appear as normal files/dirs they could add this to their vimrc:

    hi link NERDTreeLinkFile NERDTreeFile
    hi link NERDTreeLinkDir NERDTreeDir
    hi link NERDTreeLinkTarget ignore
separate out syntax matching for Dir Arrows vs old style
Merge branch 'better_symlink_highlighting' into update_highlighting

Conflicts:
	syntax/nerdtree.vim
simplify the syntax matching code slightly
remove the NERDChristmasTree option

If users want to customise the tree colours, this can be done trivially
in their vimrc. e.g.

    hi link NERDTreeFile error
fix highlighting for readonly files
trivial highlighting update for consistency