attach font changer only to font radio buttons
attach font changer only to font radio buttons

The function to set the font family was being attached to all input
elements in the entire document which included the font-size selector.
The result was that changing the font size would strip the font family
class off the body element and replace it with nonsense like
`class="config-font-family-18"`. This meant every time you adjusted the
size, your font pref disappeared and hand to be reset.

Fixes #20

digitalbiblesociety/browserbible-3,unfoldingWord-dev/uw-web,explorigin/browserbible-3,kidaa/uw-web,unfoldingWord-dev/uw-web,kidaa/uw-web,digitalbiblesociety/browserbible-3,digitalbiblesociety/browserbible-3,explorigin/browserbible-3,explorigin/browserbible-3,kidaa/uw-web,unfoldingWord-dev/uw-web,unfoldingWord-dev/uw-web,kidaa/uw-web,kidaa/uw-web,unfoldingWord-dev/uw-web