Add preliminary support for Dartlang syntax highlighting

Preliminary support for <a href="https://dartlang.org">Dart</a> has been added to address Issue #371. Following the suggestions recieved in the issue I used Java as a base and added keywords from Javascript.

Upon comparing to Dart Editor and Chrome Dev Editor with the examples from the official site as well as those mentioned in the current <a href="http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-408.pdf">ECMA spec for dart</a> it appears to follow the conventions properly. I'll be reading over the ECMA specification more throughout this week to ensure the correctness, but any assistance from experienced users would be appreciated.

This is my first merge to geany, so if I made any blunders please let me know and I will happily correct them.
I didn't see Java/etc listed, and only added it because I saw some other semi-recently added languages such as Go and Rust.