Generate PNG icons from SVG

[gmarty]

We need a tool that generate PNG for all resolutions (1x, 1.5x, 2x, 3x and 4x) from SVG images. That will allow us to iterate faster on UI implementation.

I found rebar but it seems to be a big chunk of tools, so maybe we can just build our own script using Inkscape via the command line.

Also final PNG files must be optimised, ideally with ImageOptim. So it would be nice to add this as a step in the script.

[wilsonpage]

This sounds like a good idea @gmarty.

Although I don't think we should invest too much time in this. We're not loading these assets over a network, so size is not that much of a concern. The only benefit is a (probably) marginal reduction in the amount of memory/cpu consumed rendering a @2x over a @3x image.

Let's work quickly and not prematurely optimize.

[wilsonpage]

Comment also applies to #28.