Don't build the stream extension module by default
Don't build the stream extension module by default

This commit introduces the PYLZ4_EXPERIMENTAL environment variable
to the setup.py file. If this is set to `1` or `TRUE` when setup.py
is executed then the experimental modules are built. Otherwise they
are not.

This commit makes the stream module an experimental module.

python-lz4/python-lz4,python-lz4/python-lz4