Modern Creek

• Added ability to use VS 2017 latest language version.

Thanks to @kabua

Fierce Tea

• Fixed bug where program would fall over if no PropertyGroup existed.
• Added ability to turn off cleaning.

Thanks @andytaw!

Dead Screwdriver

Added error codes, keep existing LangVersion element.

Black Sticky Boomerang

Added support for default LangVersion value.

Homeless Skunk

Just run LangVersionFixer <folder-with-csprojs> <lang-version-number>