support for captions in photobrowser, and custom external links

photo list can now accept an optional object with url and caption properties.

example in kitchen-sink.

it is also possible to pass an array of custom link attributes. either referencing by CSS class, or rel attributes...

example:

```javascript
 var myApp = new Framework7({
     externalLinks: ['off-site', 'external'],
 });
```

that will match links with either class or rel value of any value in the array.