Allow for base64 encoding

[kayneb]

While I 100% agree that base64 encoding of SVGs is less ideal than just embedding the escaped SVG XML, it would be convenient if we had a way of using base64, while taking advantage of the 4096 byte limit.

Because of #81 it is not possible to use a single webpack config for SVGs across both CSS and JS. This is inconvenient as it requires our developers to remember to prefix every JS import with jsSvg (which will set noquotes to true). If the SVGs were encoded to base64, noquotes can be set to true across the board, meaning that there would be no extraneous quotes in JS imports, and CSS handles base64 encoded strings that aren't wrapped in quotes. Until the issue in the CSS loader is fixed, this is the ideal solution in our eyes.