Added a Plugins page.

This function is already defined in changelog.html, so I think we should move it (and other common functions) to a separate file that gets included on both pages.

It's also a good idea to define a separate namespace to avoid conflicts with other libraries' functions and variable names. e.g.

```JavaScript
var picard = {};

picard.htmlEntities = function () {
    ...
};
```

So, we only create one global variable on the page (`picard`).