Makes boost.asio and boost.filesystem as an optional dependency

Adds two macros that allow disabling Boost.ASIO host conversion and Boost.Filesystem path conversion.

NETWORK_URI_HOST_CONVERTER_BOOST_ASIO: When set to zero, will disable Boost.Asio dependency
NETWORK_URI_HOST_CONVERTER_BOOST_FILESYSTEM: When set to zero, will disable Boost.Filesystem dependency

If not set, config.hpp sets them to 1 by default.