Extend support for offset notation as in "*/10+2"

[amakarishev]

We've been using cron-parser in our product for more than three years. We recently extended our cron expressions with "standard" way of using offset as it is supported by more than one schedulers. Here is how drupal describes it:

https://www.drupal.org/node/1666964
The plus ('+') operator (called "offset") can be used as an offset to a given range. For example, "*/10+2" in the minute field is equivalent to "2,12,22,32,42,52".

As of now, cron parser considers above expression invalid and fails to parse it. Can you please consider extending the parser and support +M notation when it is coupled with */N expression as above?