Don't drop RegExp flags and add url callback

Hi,

I needed to use the case-insensitive flag "i" in my RegExp, but unfortunately this flag was dropped. I fixed this issue and adapted the tests.

Also, I wanted to make a custom URL out of my RegExp match. I added an optional argument to add a callback, which generates a URL out of the match-object. Again, I changed the tests to reflext the changes.

Edit: I had to use prettier to format the ASTs, that introduced some mintor formatting changes as well. Sorry for that.