Add /shrug command
Add /shrug command

Certain popular web-based chat systems support a `/shrug` command which
appends the ¯\\_(ツ)_/¯ text emoticon (kaomoji) to a message. I've often
found myself wishing Riot had something similar, so I decided to add it
myself. (If Riot ever gets a user-extensible /-command system, this will
probably no longer be necessary. But since it doesn't have that, here I
am). Doesn't support markdown / rich text messages because I didn't want
to dig that far into Riot/the SDKs for my first PR; if someone else
would like to add that, by all means.

Signed-off-by: Andrew Chronister <52018ef6a64fdefbf8127ede4f29a9094de1c755@chronal.net>

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