Show warning when a view's size is likely to cause plugin rendering problems

[texodus]

For e.g. the Highcharts plugin, because the plugin renders the entire contents of the view, certain configurations can cause the browser to become unresponsive as the HTML/SVG/Canvas parts are rendered, even though Perspective itself may not similarly struggle. We should have a plugin-defined threshold that tells the viewer to display a user-overridable warning when the view is too large, rather than naively render.

[texodus]

This has been fixed in #290 and will be in the 0.2.4 release