Introduce optional but standard css file for plugin related css

[henrikingo]

impress.js doesn't require any particular CSS file. Each author should create their own, to achieve the visual style they want.

In practice, as we've added many plugins, a plugin will not do anything useful without CSS. (See for example mouse-timeout plugin.) The src/plugins/README.md provides example CSS that a user may want to copy paste.

It's significant enough, that it makes sense to move that CSS out into an actual css file, like css/impress-common.css. After that, all the demo presentations should be also changed to use that CSS file, and the same CSS should be removed from each of their own CSS.