Support pattern modifiers in the filter command line option.

Currently phpunit --filter option doesn't support case insensitive filtering by specifying the 'i' pattern modifier. 

Case insensitive filtering can be helpful in some situations. 
For example:
A method is named 'foo'. 
The corresponding test method is named 'testFoo'.
When we want to filter tests that test the method 'foo', we will need to specify the filter as --filter Foo instead of a more natural filter of the exact method name as --filter foo.
This limitation makes some automation code to invoke phpunit tests based on the test method selected in an IDE more difficult. 

Currently when 'phpunit --filter /foo/i  aTest.php' is executed for example, the filter is changed to '/\/foo\/i/' i.e. the code thinks that no regular expression delimiter is given. 

This change adds support for case insensitive filtering by properly detecting that a pattern modifier is given.

Example:
The following 
phpunit --filter /foo/i  aTest.php
will match a test method named 'testFoo'

Testing:
I've manually tested filters with and without regular expression delimiters such as '/' '#'.

I tried writing unit tests for this change. But testing this change indirectly through the current public interface of this class turns out to be a bigger task than I am ready to commit to. Please advice what is the best way to unit test this change if unit tests for this change is needed before this change can be accepted.