Show number of matches in find HUD

Calculate the number of matches for a find mode query, and show it in the HUD (resolves #969).

This implementation uses the number of text matches on `document.body.innerText` rather than repeated calls to `window.find` for plain text queries (see PR #1021).
In your other PR for making search work with special characters, you modified this regexp. Should this also be modified? Even better, can this logic be moved to a utility method so both completions.coffee and this can share it?