markov: Add !markov help and help_timeout config

* !markov help will display a pretty crude help message. It's funny to
  me, I dunno.
* help_timeout config option limits the number of times that `!markov
  help` is allowed to run. This is because the help message is a lot of
  lines, and it could cause problems if `!markov help` is spammed or
  abused. It can be set to 0 or less to disable.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

config.example.toml

@@ -118,4 +118,11 @@ module = "plugins.markov"
 # Additionally, users may set their reply chance between 0.0 and the value they
 # set. e.g. with a value of 0.01, a user may set their reply chance to 0.001.
 # default: 0.01
-# reply_chance = 0.01
+# reply_chance = 0.01
+
+# help_timeout
+# The number of seconds between a "help" command that's allowed. This is only
+# here because running `!markov help` sends like 10 messages in a row and it's
+# to prevent a denial-of-service attack. Generally you should not need to worry
+# about changing this.
+# help_timeout = 300