Allow registering custom fonts from a buffer

For use with jsdom package which simulates browser DOM in node.js and use node-canvas for canvas implementation a support to load fonts (truetype and openfont) from a buffer/content is needed. Because in HTML a custom font face can be specified as an URL, jsdom resolves this URL to a buffer/content by itself. So registration with buffer/content should be possible.

See https://github.com/tmpvar/jsdom/issues/744 for more details.