passing a parameter to Arrow.shift function (to check for imaginary times or not)

The shift function takes some time than absolute adding of relativedelta to datetime. If I am operating on a range where I know there is no dst change I should be able to bypass this check.

Start at the Arrow.shift function and compare its behavior with absolute relativedelta addition to datetime, focusing on the imaginary-time or DST check. Define the optional control requested in the issue, verify that it can bypass the check for ranges without DST changes, and preserve current behavior when it is not used.

Written by the indexing model from the issue text.