Add 'go to page' feature.

- Hash of URL indicate current slide index, and when hash of URL changed then show the slide which specified by hash. > This is convenient to reload but not chnge current page index at mobile device which is hard to enter space key becuse these devices has not keyboard default.
- 'Slide n of m' change from static text to drop down list.
- PowerPoint compatible keyboard shortcut: press some numeric key and enter, for example, '2','1',[Enter], then show the slide page 21.