Support to format key's value

[Xphobia]

Is your feature request related to a problem? Please describe.
Add support to format key's value when edit a key. It's more user-friendly when the value is json (or other format) .

Describe the solution you'd like
Another tool etcdkeeper may provides some ideas, it supports this feature.

Describe alternatives you've considered
Reference etcdkeeper.

Additional context
Etcdkeeper can format values.

[gtamas]

Currently I have little for this project but I will get back to it as soon as I can.

In the meantime, PRs are welcome :) If anyone wants to help, please contact me!. Comment below and mention me.

Thanks in advance!

[tzfun]

etcd-workbench can also format text