Use HTML instead of ASCII to generate tables and export with wkhtmltox

I was not satisfied with !19 (closed) since we couldn't easily manipulate styles and the implementation looked fragile (size calculations and all that), so I changed the implementation.

There's one problem though: image generation + sending of the message takes ~6s on my machine.

To solve this and other performance problems of the bot, I want to cache quotes on the database (see #13 (closed)).
So when the periodical quote request happens, besides updating the database, It'll also update the table image saved to the filesystem, when users request it (and when the bot notifies subscribers) we'll serve it directly from the filesystem.

• Implement #13 (closed)
• Plug into the periodical quote update and also update the image
• Serve image to notification subscribers
• Tests
• Configure CI to install wkhtmltopdf