Added another NERDTree vim mapping

NERDTreeFind is very useful, pops right to the current file. To reduce
lag when just openning NERDTree, I changed the mapping to "dt". Also
changed to toggle, making closing a little easier in complicated setups.
Standardized some shell style settings.

Use a shiftwidth of 2. Expand tabs. Don't use the "function" keyword, as
some shells do not support it.

Updated all of shell_functions/, along with .zshrc and .bashrc.