Custom SPARQL functions

• Add the support for declaring custom SPARQL functions, thanks to @dwhitney pull request #13
• Rework of the RDF terms manipulation API, to ease the development of SPARQL functions

What's Changed

• protect access to value by @JuniperChicago in #11
• Support for more advanced join algorithms by @Callidon in #12
• adds support for custom functions by @dwhitney in #13
• Ease creation of custom SPARQL functions by @Callidon in #14

New Contributors

• @JuniperChicago made their first contribution in #11

Full Changelog: v0.3.2...v0.4.0