Review of get_file_by_url()

Review of get_file_by_url()

Please note that this patch also introduce
which is in charge of converting a domain in a line into
IDNA and/or UTF-8 format.

Also note the introduction of BeautifulSoup() which helps
us to decode data from the downloaded URL.

Fixes (issue(s)/protocol(s) I was able to reproduce):
 * https://github.com/StevenBlack/hosts/issues/514#issuecomment-368932152

Possible fix of (issue(s)/protocol(s) I wasn't able to reproduce):
 * https://github.com/StevenBlack/hosts/issues/514#issue-300048106
 * https://github.com/StevenBlack/hosts/issues/494#issue-296166492
 * https://github.com/StevenBlack/hosts/issues/420#issue-267453114
 * https://github.com/StevenBlack/hosts/issues/372#issue-246927047
 * https://github.com/StevenBlack/hosts/issues/382#issuecomment-322010562