Support for triggering the resize of textarea when the plugin is inialized

A new `onInitialize` option *(defaulting to `false`)* will call `resize()` internally when the plugin is initially called for some `selector`.

This allows a textarea with a default value that 'taller' than the height of the textarea to be resized without requiring an input from the user.

I haven't updated the README or minified version; this is just functionality I needed an thought I'd pass the idea along.

Thanks for a the plugin!