Add variable font size in Sonic-PI

[rbnpi]

The large font is good to start with, but once you get long lines or add comments it can get annoying and make it more difficult to read a program,without pasting it to a separate editor. Add the option to set a smaller font size.

[samaaron]

Happy to consider a pull request adding this feature...

[samaaron]

This is now fixed in RC3: http://sonic-pi.net/get-v2.0