Custom route converters for hex and oct

Registers Werkzeug converters for register routes that convert path
elements from hex and oct values in a similar manner to the built in int
converter.
Allow seed to be provied as hex, oct or int

Index, slugline and api endpoints accept seed values in base 8, 10 and
16.
Document url seed parameters on TwoSlug page

Include documentation for seed parameters and various base format that
can be used.