Add tooltip support for image/audio data about hover

[phurwicz]

015b9de added a simple autoplay audio div, and checked that the existing image div is functional.

Because images/audios are files rather than simple strings, one would need to put http://<url-to-image.png> or http://<url-to-audio.mp3> in an image or audio field of the dataframe, respectively. file:///<path-to-file> could work in principle, but modern browsers block this for security concerns.

However, right now there are no high-level APIs (like hover.recipes) that make image/audio tooltips easy to use. Manually creating BokehXXXExplorer and passing tooltips=bokeh_hover_tooltip(...) can be too steep of a learning curve, not to mention having to account for everything else under the hood in recipes, such as link_selection or subscribe_update_push.

Leaving this issue open until the feature becomes more accessible.

More resources that could be useful later on:

• audio attributes / events guide
• adding delays to autoplay

[phurwicz]

Implemented and released in 0.7.0.