`ipfs update` subcommand to print out latest version #

Should check equinox and print out only the latest version number.

So, this is human friendly:

```
> ipfs update check
Current Version: 0.1.5
New Version: 0.1.6
```

This is script friendly:

```
// select one of these:
> ipfs update check --version-only
> ipfs update check --number-only
> ipfs update check --number
> ipfs update show-latest-version
0.1.6
```
