Add (re-implement) support for custom Similarity(s)

[mocobeta]

Actually, I have dropped this feature when porting Thinlet UI to JavaFX (later, Swing).

It may be possible to load custom Similarity(s) for testing on Search tab.
Each Similarity class should have different constructor parameters, so suitable constructor discovery is needed for this functionality.

Also, Search tab may need supports for custom QueryParsers (#25) / Collectors / Scorers. (Those should be discussed in separated issues.)

[mocobeta]

For contributors: Now Luke is a part of Apache Lucene. Please create an issue (if there is not exist) to Lucene Jira if you want to help with this.