[Fix #1232] make the behavior of AndOr cop configurable

Currently, the AndOr cop bans the use of keywords 'and' and 'or'
completely. This patch adds a configuration option `OnlyCheckConditionals`
which when set, restricts the AndOr cop to only conditionals.
I think you can use `Util#on_node` to recurse through the node looking for `:and` and `:or`. Correct me if I'm wrong.