Support custom Codecs

[mocobeta]

There is a method to load custom codecs.
https://lucene.apache.org/core/7_5_0/core/org/apache/lucene/codecs/Codec.html#reloadCodecs-java.lang.ClassLoader-

Users scenario (an idea):

• Click "Load custom codecs" button or link on the "open index" dialog.
• Select JAR files including custom codecs on a file chooser dialog.
• Press "OK" (then Luke loads the jars, and calls reloadCodecs() method.)

[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.