Write html to arbitrary stream, not filename

[NoahAmsel]

Currently, the write_html method accepts the name of a file and writes the HTML representation there. I'd like to be able to write to other kinds of streams besides files, for example:

from pyvis.network import Network
import sys
nt = Network("500px", "800px")
nt.write_html(sys.stdout)

Perhaps write_html can accept a file object instead of a string. For convenience, the show method could still take a string as before. Alternatively, write_html can check the type of its argument. If it's a string, the function behaves as it does now. If its a file object, the function writes to it.

Thanks for the great package!