1.6.0

This release of string_view lite adds configuration via the Tweak header mechanism as proposed by Colby Pike (@vector-of-bool).

Additions:

• Add 'Tweak header' support via nonstd/string_view.tweak.hpp

Fixes:

• Fix usage of builtin for MSVC 1920/14.2 (VS2019) for pre-C++17
• Fix to use nssv prefix in configuration in t.bat, tc.bat, tg.bat batch files in test/

Changes:

• Add Visual Studio 2019 to the Appveyor build matrix (#39)