Exposing the spans of matches in the API

[matanox]

Wouldn't it be nice to add in the API the option to get the spans of the matches, in addition to just the matches, in the spirit of these python and Java implementation?

https://stackoverflow.com/a/250303/1509695
https://docs.oracle.com/javase/7/docs/api/java/util/regex/Matcher.html#start()

Or is this already baked in and I missed it in the source or unit-tests code?

[matanox]

In case the PR is accepted, I suggest usage of the new method could be exemplified in the README.