Enable Byte-Range headers in HTTP requests when targetting HTML5 when…

… environment variable EMSCRIPTEN_WGET_HEADERS is defined.

This extension is not part of Emscripten yet, so it requires a custom build. In order to keep backward compatibility, it is necessary to define the environment variable EMSCRIPTEN_WGET_HEADERS to 1 in the host OS to enable custom headers when calling emscripten_async_wget2_data.

Merge branch 'dev'