fix --multi hash decoding and add saner options

[riknoll]

fixes microsoft/pxt-microbit#5397

as noted in that issue, the way to use the --multi page was to include a hash like this:

makecode.microbit.org/--multi#lefthash:|:righthash

where lefthash will end up being applied to the left iframe and righthash to the right iframe. however, because the delimeter includes the character |, it was often getting encoded automatically as %7c which breaks the parsing. this fixes that bug by calling decodeURIComponent on the hash to make sure all the percent encoded characters are parsed.

still, this is imo a silly way to structure that URL, so i've also added a new format that is easier to use. you can now add arguments using query string syntax like so:

makecode.microbit.org/--multi#left=lefthash&right=righthash

or, if you're just referencing two scripts, you can do this:

makecode.microbit.org/--multi#left=leftscript=_xxxxxxxxx&rightscript=_xxxxxxxxx

which will automatically add the pub: prefix to the hash for the iframes

[srietkerk]

LGTM