Fix the "m" flag in regexps for correct multiline behavior (#138)

When the "m" flag was present in a regexp we were setting Joni's
Option.MULTILINE when we really should have been negating Joni's
Option.SINGLELINE. The naming of the options in Joni is confusing when
compared to the options used in JS regexps.

Fixes #138
Merge pull request #139 from bbrowning/138

Fix the "m" flag in regexps for correct multiline behavior (#138). Thanks @bbrowning!