Provide a tool to download fonts from Google Fonts and add relevant go embed and font loading statements

[kkoreilly]

This would allow developers to use more non-standard fonts in their apps without having to worry about the user having them on their system.

[kkoreilly]

It makes the most sense to do this by embedding fonts into executables, so a simple goki fetch font FontName command in goki/packman that downloads fonts, puts them in the right place, and adds the code to embed and load them would be the easiest way to implement this.

[kkoreilly]

The Noto fonts will be available for apps to use by #568, but developers may want more fonts, so we should still probably do this.

[kkoreilly]

We should also support a similar thing for icons and support generating constants for icons.

[kkoreilly]

See #849.