Custom language loaders for specified file extensions

 Allows one to write custom language loaders for specified file extensions
 to be hooked into require.
 .
 This supports the creation of DSLs having a syntax that is most
 appropriate to their purpose, instead of abusing the Ruby syntax.
 .
 Required files are attempted first using the normal Ruby loader,
 and if that fails, Polyglot conducts a search for a file having
 a supported extension.