rstantools v2.6.0

• Do not open project on error by @karchjd in #145
• New pkgdown theme by @VisruthSK in #141
• Remove C++14 from SystemRequirements by @jgabry in #150

New Contributors

• @karchjd made their first contribution in #145

Full Changelog: v2.5.0...v2.6.0