Plugins cannot be loaded if using wintersmith programmatically

[ajpiano]

So far as I can tell, there's presently no way to load custom plugins if you're invoking wintersmith from within a script. If you pass them in as options, they are simply ignored, as the code to handle loading plugins only exists/is invoked within the cli interface. I'll be putting something together to attempt to make this work in the next few hours and will send a PR assuming I do!

[jnordberg]

Fixed