ipfs update command

[jbenet]

As discussed in #30 and #31, IPFS should be a self-updating tool. I think this should happen with:

• an internal update routine (probably using @inconshreveable's go-update
• a manual ipfs update command, described below.
• optional auto-updating.

ipfs update

It should work like this:

# update to a specific (semver) version
> ipfs update <version>
> ipfs update 1.2.3
Your old version was: 1.2.2
Updated to (latest) version: 1.2.3      # (latest) only if actually latest

# update to latest minor/patch version
> ipfs update   # (if newest version is backwards-compatible)
Your old version was: 1.2.2
Updated to latest version: 1.2.3

> ipfs update   # (if newest version is backwards-incompatible)
Your old version was: 1.2.2
Updated to latest (backwards-compatible) version: 1.2.3

Note: There is a newer (backwards-incompatible) version: 2.0.0.
Run with the --latest flag to update to this version.

# update to latest version
> ipfs update latest
Your old version was: 1.2.2
Updated to latest version: 2.0.0

# list available updates, but don't update
> ipfs update check
Your current version: 1.2.2
New version available: 1.2.3

> ipfs update check
Your current version: 1.2.2
New backwards-compatible version available: 1.2.3
New backwards-incompatible version available: 2.0.0

> ipfs update log
# ipfs update log

## 1.2.3

This version includes a few simple bugfixes.

* abcdfre - fixed bug in cmd parsing
* 123abcd - fixed bug in http serve

## 1.2.2

...

Flags (these are nice-to haves, here for doc purposes):

• --dry-run don't actually do it. go through the motions but don't actually install update. useful for testing. (maybe, not a priority)
• --key provide a different key for checking signature
• --ignore-signature don't check signature

The cmd help should be:

  UsageLine: "update [<version>]",
  Short:     "Update the go-ipfs tool",
  Long: `ipfs update [<version>] - Update the go-ipfs tool

    ipfs update           - Update to latest (backwards-compatible) version
    ipfs update latest    - Update to latest version
    ipfs update <version> - Update to a specific version
    ipfs update check     - List new versions available
    ipfs update log       - Shows the update log

This command updates the ipfs tool itself. It downloads new versions from the
internet, checks digital signatures, and takes care of the installation.

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