Add app destroy command

[bgins]

Summary

Problem

Currently there is no way remove a published app once you have published it with the Fission CLI.

Impact

This can cause unnecessary data to be stored on Fission's IPFS nodes, costing resources that didn't need to be spent.

Solution

Add a command to the the CLI that runs IPFS command to remove files.

Details

We want to add this feature during the CLI rewrite in Rust.