Use best-matching version

### Steps to reproduce

```
$ asdf list ruby
> 2.3.1
2.3.7
$ asdf local ruby 2.3
```

### Expected behavior

Use version 2.3.7, since it is the highest version that matches the given value.

### Actual behavior

`version 2.3 is not installed for ruby`

### Environment

**OS**: macOS 10.13.6

**asdf version**: 0.5.1

Similar to #226, I'd love to be able to be less specific about versions and get the highest matching installed version. For something like postgres, I might just say `9` or `10`, for ruby, I might use major and minor. I think this would make the tool a lot more useable.

If this sounds good, I could probably put together a PR, although I'm not sure how much overlap there will be in the logic for installing a highest-matching version.

Start by reproducing the behavior with `asdf list ruby` and `asdf local ruby 2.3`, then trace the CLI path that resolves a requested version. Compare that resolution with the installed versions and verify that partial requests select the highest matching version while exact requests retain their current behavior.

Written by the indexing model from the issue text.