New feature: introduce compressor registry

[mpenkov]

This feature makes it easy for people to transparently decode files not supported by smart_open out of the box.

For example, you can now do:

def _handle_lzma(file_obj, mode):
    import lzma
    return lzma.LZMAFile(filename=file_obj, mode=mode, format=lzma.FORMAT_ALONE)

import smart_open
smart_open.register_compressor('.lzma', _handle_lzma)
with smart_open.smart_open('file.lzma', ...) as fin:
    pass