Support for custom regular expression for number_to_delimeted

The format for placement of delimiter varies in currencies like INR, where instead of `123,345,455.00`, the number would  be `12,33,45,455.00`. This adds support to allow user to provide custom handling of how the placement of delimiter is derived.

Changes :
- Extracted `DELIMITED_REGEX` to `delimited_regex` method and made use of user passed `options[:delimited_regex]` if available. Changed `DELIMITED_REGEX` to `DEFAULT)DELIMITED_REGEX` to signify what it means.
- Added tests for number to delimited and number to currency in both actionview and activesupport.