Add commands for resizing

It seems you can actually create functions in Vim. You just
have to use function! command. You can actually pass parameters
and do things using these parameters. Not bad.

You can't set a parameter to a setting (for instance, you cannot
do set columns=a:width. You can use let, though, using an
ampersand (&) symbol and the setting you are trying to set.
For instance, let &columns=a:width. This will work.

I define a function for resizing GVim. Will set columns and lines
using a single command. I also define three commands for setting
some standard sizes: big, small and medium.
Install NERDTree