Add substring action

[parmentf]

For the moment, if you want to extract the month from a string "20150310", you have to mess withtruncateandshift`.

{
  "truncate": 6,
  "shift": 4
}

Maybe a substring: [offset, length] action would be more user friendly:

{
  "substring": [4,2]
}

[parmentf]

89d9943 made it.
Will be available from v3.4.0+