Allow for specifying the indent character to be used and how many (tabs OR spaces!)

[thatsmypasswd]

Hello,

We are using sops as part of a pipeline that originally generates json pretty-formatted files that are space-indented. Unfortunately when we then use sops, it decides to indent using tabs, which can cause our diffs to be large and noisy.

We'd like sops to allow defining an indent character of choice and how many of those characters, similar to many other frameworks that work with JSON.

[thatsmypasswd]

Looks like #1028 was having an issue with tabs/spaces as well.