Option to disable exclamation mark syntax.

I try to follow "The code reads well if the successful flow of control runs down the page, eliminating error cases as they arise." from https://golang.org/doc/effective_go.html#if
It's more lines, but I find it more readable as it avoids an additional indentation level:

c, present := s.cmds[words[0]]
if !present {
  return
}
direct = direct && c.Pub
indirect = indirect && c.Pub && !c.NoExclamation
private = private && c.Priv
if direct || indirect || private {
  go c.Handler(&Event{Bot: b, Line: line, Target: target, Args: args})
}