regex tool: add option for use in string templates

[rgerhards]

These require a bit different escaping than the traditional $template string (which actually did the escaping wrong...).

[Ralnoc]

I give a big +1 to this. I use the regex tool extensively and have often wished there was a way of generating regex for use in string templates. This is especially true for when you need to generate regex for things like the re_extract() function which, like string templates, uses a slightly different escape configuration to the %property% based regex.