add index parameter to `multisig raw cat`

[andynog]

If there are multiple transactions in a particular [chain]/[keyname] it's hard to see the content of a particular transaction. A nice option would be to have an index parameter (-i or --index) that would show only the transaction content on that index.

[ebuchman]

I believe you can do this already using the path ie . multisig raw cat juno my-key/1

[andynog]

Looks like it's possible, but probably need to document this option because I could not infer from the README or help usage