Maintain case-insensitivity when merging multiple matchers (#213)

When given two regexps, Regexp.union preserves the options set (or not
set) on each regex; this meant that none of the multiline (m),
case-insensitivity (i), or extended syntax (x) options were set.  Our
regexps are written expecting the m, i, and x options were set on all of
them, so we need to make sure that we preserve that behavior.