Support position to context.xml in Query Elevation Component

Query Elevation Component is useful but is kind of limited.

Usually we want to boost one document for one query, but not necessary put at the first one.

For example, user searches walking dead - we want to boost our shows "Deadly Vampire", but we don't want to show it as the first one - as that may piss off the user.
We want to show "Deadly Vampire" at the 2nd or maybe 3rd, 4th position.

Seems at Editorial Query Boosting Component https://issues.apache.org/jira/browse/SOLR-418, the original draft implementation actually supports it - the priority property.