Fixed reload of slide with a custom ID containg special character

In ID selector some reserved characters like ., #, {, }, ... must be
escaped. It's easier to use getElementById() method which doesn't require escaping.

The current reveal.js will not reload deck at specified slide if it has ID containing for example period, like <section id="R1.2.3"> - this will be turned into #R1.2.3 selector, which is invalid.

Some issues and complexities of escaping CSS identifiers are availabe here https://mathiasbynens.be/notes/css-escapes