Show version numbers for toolchains

`multirust list-toolchains` or possibly some other command should really show the version number for each installed toolchain. i.e.

```
beta (1.8.x)
nightly (1.9.x)
stable (1.7.x)
```

This would be very handy indeed.

Start by running `multirust list-toolchains` and trace the command entry point that formats installed toolchains. Check how each toolchain is identified and where its version can be obtained; done means the listing includes versions in the requested form for beta, nightly, and stable toolchains.

Written by the indexing model from the issue text.