Add support for multiple ranges in a host

e.g.  db[01:10:3]node-[01:10]

- to do this we split off at the first [...] set, getting the list
  of hosts and then repeat until none left.
- also add an optional third parameter which contains the step. (Default: 1)
  so range can be [01:10:2] -> 01 03 05 07 09