[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.
[Fix #1227] Don't permanently change yamler

It can affect other apps. So we still change it to avoid
problems with empty yml files, but we change it back.
Merge pull request #1239 from jonas054/1227_yamler

[Fix #1227] Don't permanently change yamler
Merge pull request #1236 from vrthra/fix/1232/configurable_andor

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