Fix NERDTree installation check.

Since we now override a NERDTree dict, we need to check if that dict
exists, not some other related function.
ZSH: Bind Ctrl ( + Shift ) + Left / Right keys

Two variants are bound: simle Ctrl+Left will jump to the begin of the
previous word even within quotes (similar for Ctrl+Right) whereas Ctrl +
Shift + Left will stop on words separated by '-/_', nice to fix an error
in a long path.