Addition of regex flags

[emahuni]

Summary

Ability to set, usually necessary, regexp flags. It may not be all, but just basic ones.

Motivation

Attempting to search for the following regexp '.*?version__' when it's known that it may have different case leads to a regexp like so '.*?(?:version|VERSION|Version)__' instead of just '.*?version__' with the "i" flag for a non-case-sensitive pattern.

Describe alternatives you've considered

const Oni = OnigRegExp (`.*?version__`, 'i');