External warning label for headers in MSVC

Currently the guide recommends against `/Wall` for MSVC because the system headers do not comply. https://blogs.msdn.microsoft.com/vcblog/2017/12/13/broken-warnings-theory/ has an experimental feature that lets you turn off warnings for headers outside of your control.

`/external:anglebrackets` – a switch that allows a user to treat all headers included via #include <> (as opposed to #include "") as external headers

Might be good behavior to use the above switch assuming your MSVC compiler supports it.