Add support for RDF turtle linter and fixer

**Name:** turtle
**URL:** https://www.w3.org/TR/turtle/

For a linter the command line **ttl** tool can be used which provided both errors and warnings:

https://github.com/IDLabResearch/TurtleValidator

For fixing the command-line utility called **rapper** from the Raptor toolkit can be used:

rapper -i turtle -o turtle
http://librdf.org/raptor/rapper.html

I have little experience with vim coding but will have a look around to see what I can do in regard to a pull request. Any existing work or advice appreciated.