Add support for case-intensive search

[anuragkh]

The current search() implementation performs a case-sensitive match. We should add a flag to the search call to make it case insensitive, i.e.,

def search(query: String, caseInsensitive: Boolean = false): RDD[Long]

For a query string abc, it would be equivalent to executing the following regular expression query:

[aA][bB][cC]