Add JSON output option to 'prepare-release' command

[ap0llo]

This PR adds a --format parameter to prepare-release that allows setting the output format to json.

I made some adjustments to the output format proposed in #481.
The output will look like this:

{
  "CurrentBranch": {
    "Name": "master",
    "Commit": "5a7487098ac1be1ceb4dbf72d862539cf0b0c27a",
    "Version": "0.3-pre"
  },
  "NewBranch": {
    "Name": "release/v0.2",
    "Commit": "b2f164675ffe891b66b601c00efc4343581fc8a5",
    "Version": "0.2"
  }
}

• The output includes the id of the latest commit on the respective branches. This way, the exact version on the branch can easily be queried using get-version.
• The property names use pascal-case instead of camel-case so the output is consistent with the JSON output of get-version
• The properties devBranch and releaseBranch are namedCurrentBranch(the branch the command was started on) and NewBranch (the new branch that was created) instead.
  That way, the scenario that the command is started on the release branch can be handled as well. In that case, the NewBranch property will be null because no new branch was created.

Fixes #481

[AArnott]

The property names use camel-case instead of pascal-case so the output is consistent with the JSON output of get-version

Your sample actually shows PascalCase. So I'm confused as to which one is accurate.

[ap0llo]

The property names use camel-case instead of pascal-case so the output is consistent with the JSON output of get-version

Your sample actually shows PascalCase. So I'm confused as to which one is accurate.

Sorry about that, I wrote that down in the wrong order.
I fixed the PR description

[AArnott]

This looks great. Very well thought out and I'm glad you did it rather than me because you took care to handle scenarios I wouldn't have thought of.
Just a couple of nits to take care of before I merge, please.