add 'pinch to zoom' mode

[gka]

For some charts we just have to acknowledge that there is no way to make a 300px version for phones. instead, we need an option to display a zoomed-out version of the desktop chart with a note saying "pinch to zoom".

ai2html should be aware of this option and display the note and add this code snipped to re-enable pinch-zoom:

<script>
viewport = document.querySelector("meta[name=viewport]");
viewport.setAttribute('content', 'width=device-width, initial-scale=1.0');
</script>