Add ability to exclude certain characters from conversion

## Summary
* Added an optional parameter `chars_to_ignore` to the `convertKana` function
* This parameter is a String containing zero or more characters to be excluded from conversion
* Every character in the `chars_to_ignore` String will be excluded from conversion if that same char is found in the `original_string` input string parameter.
* If the parameter is an empty String or not passed, no exclusions will be done (normal operation)
* Added and updated unit test code accordingly to test this exclusion logic.

## Relevant Issue
#2