added utilities for URI slug generation

this allows for converting arbitrary strings to URI slugs:

    label = "hello world"
    IqRdf.send(label, IqRdf::Skosxl.Label)
    # results in invalid data: `:hello world a skosx:Label.`

    label = "hello world"
    IqRdf.send(IqRdf::Origin.new(label).to_s, IqRdf::Skosxl.Label)
    # results in valid data: `:helloWorld a skosx:Label.`

adapted from iQvoc:
https://github.com/innoq/iqvoc/blob/0cfa829283da64dc4035b5a1c9ada255a1407bce/lib/iqvoc/origin.rb

arguably IqRdf should do this automatically - or at least validate URIs
accordingly?

however, perhaps this iQvoc `Origin` is overkill and URL-encoding
(i.e. `:hello%20world`) suffices?