youtube_title.tcl: handle HTTPS and youtu.be URLs; use standard JSON parser

YouTube started defaulting to HTTPS some time ago so share
URLs which made the youtube_title.tcl no longer match pasted
links.  Add “s?” to the pattern.  While at it, modify it to
also support youtu.be URLs as well as the full youtube.com URLs.

At the same time though, the pattern is now more strict which,
however, should not negativelly affect anything, but instead
prevent the script from matching arbitrary links such as
evilyoutube.com.

Finally, remove hand written JSON decoder in favour of TCL's
standard json package.  The decoder had some bugs and some
JSON data was not properly parsed.  Switching to the json
package not only makes the code shorter, it also get rid
of that bug.