Add test for ignoring text before the command

For example, when parsing "1 2 3 !sum 2 2" the result should be 4.
Use split-sequence instead of a handmade function
Use let* to make handle-message more readable
Fix trying to send a nil message

cl-irc:privmsg doesn't know how to deal with a nil message, and a empty
string is ignored, so when nothing matches, return an empty string.
trivial: Fix indentation
New commands !help, !s-u
Enable ssl by default