Enable image capture for file uploads

[jayohms]

Addresses: #139

Given the following markup, turbo-android already handles opening a file chooser to upload files:

<input type="file" id="file" accept="*/*" />

However, it's not currently possible change the file Intent to capture an image from the camera. Android's WebChromeClient handles the capture attribute:

<input type="file" id="photo" accept="image/*" capture="user" />

This PR adds the following behavior:

When set on a file input type, operating systems with microphones and cameras will display a user interface allowing the selection from an existing file or the creating of a new one.

When capture is enabled, Android will present an Intent chooser:

Known limitation: video and audio intents are not currently supported, but this solves the primary use case.

[lateplate]

Nice, LGTM!