fix non-utf8 channel name handling

[any1one]

when backing up a channel with chinese characters, got his error :

Traceback (most recent call last):
  File "slack_export.py", line 369, in <module>
    fetchPublicChannels(selectedChannels)
  File "slack_export.py", line 126, in fetchPublicChannels
    print("Fetching history for Public Channel: {0}".format(channel['name']))
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-1: ordinal not in range(128)

encoding channel name to utf8 fix it

[rueycheng]

Unfortunately this patch doesn't work with non UTF-8 channel names. For example, if you fire off the export script with --publicChannels general you'll end up with two directories, b'general' and general, rather than just one. JSON files are dumped to the first directory, leaving the second empty, which would later trigger a 500 error when you fire up slack-export-viewer.

Undoing this PR solves the problem on my side (python 3.7.4, macos 10.14.6)

[jcul]

Yes, this pull request creates lots of duplicate folders.

[zach-snell]

Feel free to make a pull request with an adjustment or a rollback and I will merge it though.
Thanks!

I have not worked on this in a while since it has limited interest but am happy to merge in valid adjustments.

[jcul]

I submitted pull request #20 to undo these changes until a better solution is found.
I did some tests with unicode characters in channel names and didn't experience the same issues.