Input Text updates

I made some modifications to the way input text works:
I started by making it it's own tag (<input_text>), so that input specific parameters wouldn't be confusing on regular text. This did not remove the ability to use input="1" on a text for backwards compatibility.
I also added password_mode as an option for input text. This would trigger the corresponding passwordMode boolean on the widget, displaying * instead of the text.
Finally, I updated the README.md to reflect these changes.

This is the first time I've done a pull request, so please let me know if there's any changes/modifications I need to make. 
Yeah, but to someone who looks at a flixel-ui xml the first time, single-letter-abbreviations won't really have any meaning. It's not like there's a major difference between `a` and `alpha`, for example. Personally I prefer readability, xml just happens to be a bit verbose, but that at least means it's not toally cryptic.