Refresh slide dimension settings after each resize

Currently _bespoke-scale_ saves slide dimensions at initialization and refers to it through further process, it doesn't work well, if for some reason we changed slides size during the presentation.

I have such use case when using [bespoke-notes](https://github.com/medikoo/bespoke-notes) plugin. It allows to optionally display notes aside of slide (then slide width becomes twice as long as initial one). Additionally it [dispatches `resize`] (https://github.com/medikoo/bespoke-notes/blob/master/index.js#L73) event to invoke any eventual resize related logic. Still _bespoke-scale_ doesn't pick size change, and presentation needs to be brutally reloaded to adjust to new size.

Demo of this case: http://medikoo.com/asynchronous-javascript-interfaces/ (press `n` key to toggle notes).

This change fixes that.