Allows customization of icons

[jeroenheijmans]

Fixes #245 in original repository.

After this change the user can fully customize the "icons", replacing the default html entity characters by any string, including:

• Different html entities or unicode characters;
• Bits of html offloading things to a icon font;
• Html img tags;
• Empty strings (that effectively removes the icons);

The change is backwards compatible, only "activated" if the element that uses this directive also has a custom-icon attribute. That attribute works analogous to the translation attribute. Backwards compatability is the reason for the somewhat convoluted "pad-with-nbsp" function, that might be removed in a future major release.