Support "--output=<file>" in cmds that return data to stdout

[lidel]

Problem

@aschmahmann noted that printing to stdout is not the best ergonomics when working with commands like:

• ipfs block get
• ipfs multibase encode|decode.

They take input from a file OR stdout, but return data to stdout only, which is problematic:

• awkward when using in non-posix shells like Powershell
• there is a general inconsistency: --output= IS supported in ipfs get, but not in mentioned block or dag export.

Solution

• A) simply add -o | --output= to mentioned commands
• B) (nice to have) find a more elegant way to add -o | --output= as part of https://github.com/ipfs/go-ipfs-cmds
  • we want to ensure that every place where we return data to stdout support outputing to a file
  • does not need to be fully automatic: if it is possible to detect stdout just panic and ask to add the -o --output parameter to the command definition

[schomatis]

Working on this.

[schomatis]

The go-ipfs-cmds way seems doable. Will push a prototype for review.