Tool assets conversion to HTML SVGs

[grosbouddha]

Start replacing all images by their SVG counterparts (see src/templates/svg-assets.html). SVG icons are inlined in the HTML, not loaded as external resources.

Pros:

• Easy to get high-definition icons (no more x2 or retina icons
• Easy to have colored icons (as long as they are mono-color)
• No external image loading: this will ease making Piskel an installable app (e.g., embedded in a
  as a JS lib) without any remote static asset dependencies

Cons:

• It will make the HTML document bigger and non-cacheable. However, it is still gzipped/compressed (with a reduction factor of ~3.5). The impact on the gzipped size is about 15 KB.