Canvas tiles example no canvasFunction API

[ChouGary]

Canvas tiles example only show xyz, no way to show 「tile extent、tile url、tile json url、related features number」 for tile debug .

http://openlayers.org/en/master/examples/canvas-tiles.html need api 『canvasFunction』 show custome message, like D3 sample http://openlayers.org/en/v3.10.1/examples/d3.html

[ahocevar]

Would you be willing to create a pull request? Basically all you need to do is add a textFunction option to olx.source.TileDebugOptions in externs/olx.js and use that function in src/source/tiledebugsource.js, with a default of what we currently have.