Add support for modifiers to remove leading zeros or convert to spaces

[the-mars]

The buildin strftime currently supports modifiers after the % sign. They are not documented at php.net/manual/en/function.strftime but they work on Linux and I have seen code which is using it.

For example %-d will remove a leading zero if the day of month <10. While %_d will replace the leading zero by a space. I found in a comment that on Windows %#d is doing the same as %-d, so I added it too. This will make it platform-independent ;-)

[alphp]

Thanks for the pull request.
This has made me realize that I need to do some tests to avoid breaking what already works.