Adds "replaceBaseText" option, Fixes #39

This adds a new setting ``replaceBaseText`` which will replace the original text of the element with the first string if enabled.

```javscript
$('#typer').typed({
  replaceBaseText : true,
  strings         : [
   'Text One',
   'Text Two'
  ]
});
```
```html
<div id="typer">This text will be replaced with text one</div>
```