Added session configuration option.

Simple config addition to allow selecting a saved putty session.  Useful if you have saved settings other than the default you want applied to the vagrant putty session and you don't want to pass the `-load` option on the command line every time.

Putty option parsing seems to have a bug where a hostname on the command line won't override the hostname in a saved session unless the hostname comes _after_ the `-load` option on the command line.  This forced me to add `-load` as the first item in `ssh_options`.  Not what I would have preferred, but oh well.