Support getting documentation on hover

[rictic]

Two research tasks are prerequisites for this:

• Track down the APIs for us to notice a hover and extract the line and column where the user hovered.
• Track down an API for rendering a tooltip. Ideally one that supports scroll on overflow, and rendering either markdown or raw HTML.

[rictic]

Thanks to @TimvdLippe it looks like this is what we want for getting the column and line number: https://github.com/steelbrain/linter-ui-default/blob/master/src/editor/helpers.js