Use SVG templates instead of hard-coded layout

Allow to use a SVG-based template to define the layout of the cover, so you can use your own templates (for example, an O'Reilly clone using kitties :-P ) and fill it with data from ```book.json```. This fixes issue #2.

It's using a temporal static HTTP server so ```canvg``` can be able to load the images, and I've also cleaned the algorithm to calculate the size of the title font. Probably the text sizes could be auto-calculated by the SVG engine, but seems tricky, so I've leave them scripted. It should be improved if possible, though.

Only problem seems to be Unicode characters, I'm not sure if the ones on the title were working... :-/ Except probably that issue, the templates are working :-)