Support basic string transformations

[ktk]

I'm not sure how far to go in this one as not everything is necessary in a tool like RML but I think some base string processing should be possible. What I had twice now is that I would like to have a value lowercase in the URI I construct and currently I can only do that with postprocessing.

[seralf]

it could be useful indeed to have the chance to "plug in" custom functions for manipulating literals / IRIs.