Please add option for 'text=' value in font request URL.

[a-borodko]

From https://developers.google.com/fonts/docs/getting_started*

Optimizing your font requests

Oftentimes, when you want to use a web font on your website or application, you know in advance which letters you'll need. This often occurs when you're using a web font in a logo or heading.

In these cases, you should consider specifying a text= value in your font request URL. This allows Google to return a font file that's optimized for your request. In some cases, this can reduce the size of the font file by up to 90%.

[dherault]

See #68