change plantuml_output_format to svg_img

This depends on upstream PR by melvin being merged for probably 0.12: https://github.com/sphinx-contrib/plantuml/pull/17

Currently it renders both a SVG and PNG, so PNG is used for fallback for old browser. However since IE9 (2011) and Android 4.4 (2013) SVG <img/> is supported, so this is a better default. This improves build time by over 50% for project with many diagrams, since it turns out PlantUML is faster at SVG then PNG.