Configuration to set whitespace before & after pivot for center-justified indent

[cnojima]

Describe the solution you'd like
Implement an extension configuration to enable specifying a spacing value before and after the pivot for center-justified pivots.

Additional context
e.g., with a value of 3, the following

{
  "a": "foo",
  "bcde": "bar"
}

becomes:

{
     "a"   :   "foo",
  "bcde"   :   "bar"
}