Add tests for %I and %p format options

For traits in constexpr context use ::value

* Fixes HowardHinnant#542

Remove BUILD_TZ_STATIC and replace with BUILD_SHARED_LIBS

* This is more standard CMake
* See https://cmake.org/cmake/help/v3.1/command/add_library.html

rename and mark-as-advanced the following cmake-variable used in func…

…tion print_option:

 CURR_${OPT} -> PRINT_OPTION_CURR_${OPT}
so that the date-project now has the following advanced cmake-variables:
 PRINT_OPTION_CURR_BUILD_TZ_STATIC
 PRINT_OPTION_CURR_USE_SYSTEM_TZ_DB
 PRINT_OPTION_CURR_USE_TZ_DB_IN_DOT

Update README.md

Make wide string literals in to_stream

Have get_version check for the file named version first

Update README with chrono_io.

Update README.md