Custom mode support

[zauguin]

This allows the mode pseudo-feature to be used to enforce an alternative loader, which can be used to support the same font format using different backends.
One example would be allowing OpenType fonts to be shaped with HarfBuzz through luahbtex-harf.

As a related change this adds features.raw for accessing the actually provided features without modification by defaults. This is useful for lua fonts and alternative backends which might interpret features differently.