Added 'pin ls' Command

[mappum]

This PR adds a ipfs pin ls command, where users can view a list of what is being pinned. It only shows direct pins (keys that have been added with ipfs pin add). Maybe we want it to show the indirect/recursive pins as well?

[jbenet]

@mappum why the change? i think -r, --recursive were good

[mappum]

Well we have global options with those names (for recursive file adding), and we error if we have a name collision.

[jbenet]

@mappum i think that's a design problem the commands lib has to handle. -r is a well established flag to mean recursive. clients need to be able to use it.

[mappum]

Ok, we can solve it by adding a Recursive modifier to commands.Option, to specify an option that tells the commands lib to allow recursive files (kind of like how we already have one to enable recursive files for a certain argument).

[jbenet]

How about this:

The commands library could have a set of special options which you can use, i.e. cmds.OptionRecursivePath, which the command library knows to handle specially. (This could be the same for the encoding option.) So, instead of being present everywhere, -r is only there when you ask for it. (encoding should still be everywhere)

e.g. as a user:

var cmdAdd = &cmds.Command{
  Options: []cmds.Option{
    cmds.OptionRecursivePath,
    ...
  },
}

var cmdRefs = &cmds.Command{
  Options: []cmds.Option{
    cmds.StringOption("r", "recursive", ...),
    ...
  },
}

[mappum]

Good idea, I'll implement that.

[jbenet]

Good command idea! Would be great to include an overview of how the command will work in the PR, like this one: #229 (comment)

• use -r, --recursive (4c1a5f6)
• allow showing indirect / recursive pins (9de0100)
• copy of pin set? (e6091be)

[mappum]

BTW, this is ready for merge if you want to give one last look.

[jbenet]

switch typeStr {
case "all", "direct", "indirect", "recursive":
default: 
  return nil, cmds.ClientError("Invalid type '" + typeStr + "', must be one of {direct, indirect, recursive, all}")
}

[jbenet]

LGTM! ready to merge