Accept svg file content sent by postMessage

Hi Matt,

We're following your project closely and are really excited about the upcoming svg import/export features.
Today I tried it with an svg file produced by Prototypo, I noticed some bugs in the outlines but I wanted to know how hard it would be to make it possible to open a Prototypo font directly in Glyphr Studio, with a single click.
And it was surprisingly easy! The change is only 4 lines of code for Glyphr.

I know svg isn't the best interchange format for fonts but it's already a good start. Let us know what you think and let's keep in touch.

Cheers,
Louis-Rémi

![prototypo glyphr](https://cloud.githubusercontent.com/assets/39374/4977787/7d7eb466-68ea-11e4-8759-82ea7a435e00.png)