Allow adding extra VersionMatchers in InlineIvyConfiguration

It seems that currently there's no way to add additional `VersionMatcher`s to the IvySettings configuration.
I needed such a feature to add my own matcher, and did so through a new field in `InlineIvyConfiguration` called `extraVersionMatchers`. 
I don't think there's a real need to abstract an sbt class over this and later do a conversion to Ivy, which is why I didn't introduce a new setting either (anything like `SettingKey[sbt.VersionMatcher]` seemed unnecessary).

Instead, the use case for this is to just transform the `ivyConfiguration` and add some matchers:

```scala
ivyConfiguration ~= _ map { 
  case ic: InlineIvyConfiguration => ic changeVersionMatchers (myVMatcher :: Nil)
  case c => c // or something...
}
```

Please let me know if the approach seems reasonable.