[feature-request] Show the number of direct children when collapsed

[balta2ar]

e.g. keys of an object or number of elements in an array:

{
  "eventRecords": {…} (48),
  "registrationRecords": {…} (20344)
}

(also requested here: antonmedv/fx#138)

[brewingcode]

I have this in #40, but I haven't seen @PaulJuliusMartinez around since early March, so that PR remains unmerged.

[PaulJuliusMartinez]

This has been implemented in v0.9.0.

I'm not totally thrilled with it; it's definitely useful for arrays, but I find it to be less so for objects.