Improve mailto handling and add generic value modifier to _add_triple_from_dict

[seitenbau-govdata]

The generic modifier allows to provide a function which updates the value before it is added to the graph from the dict.

Also, this improves handling of mail addresses:

• Use URIRef instead of Literals for email addresses
• ensure that mailto: is prepended to the address

https://github.com/GovDataOfficial/ckanext-dcatde/issues/6

[amercader]

This looks good @seitenbau-govdata. @metaodi raises a good point, so I think it's safer to add the new keyword param at the end.

[seitenbau-govdata]

@amercader and @metaodi Good point! I agree. I have pushed a new commit with the change.

[amercader]

Thanks @seitenbau-govdata