Fix non-standard Slack markdown syntax for bold and
strikethrough
Unescape HTML entities for Slack control characters (&, <,
and >)
Canonicalize directory paths specified as command line arguments
This fixes a bug where certain forms of the --src-dir option
value caused the inferred destination channel (if not explicitly
specified with the optional --dest-channel) to not be properly
set to the last directory in the path
A trailing slash (on Unix) caused the dest channel to be None
Ending with a relative path (e.g. . or ..) would set that to
the dest channel name
strikethrough&,<, and>)--src-diroption value caused the inferred destination channel (if not explicitly specified with the optional--dest-channel) to not be properly set to the last directory in the pathNone.or..) would set that to the dest channel name