[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.
The AST nodes are documented [here](https://github.com/whitequark/parser/blob/master/doc/AST_FORMAT.md).