Update Subs-Package.php

Source code to accept <input> tags from package-info.xml file.

Syntax and examples are noted on my main fork for SMF2.1 repository.
Other suggestions:
- add ability to accept <javascript> tags from package-info.xml
- add javascript options for input 

This will give mod authors more freedom of db manipulation during install and/or uninstall.
Update Packages.php

This fills the $context['new_inputs'] array with the necessary $_POST data from the form.
Update Packages.template.php

If there were inputs fed into the $context['new_package_inputs'] array from Subs-Package.php, this will display them.
Create Underdog-Additions-README..txt

Explanation of changes made.
Update Subs-Package.php

Added syntax for class attributes.
Update Underdog-Additions-README..txt
Update Underdog-Additions-README..txt
Update Underdog-Additions-README..txt
Update Subs-Package.php

condensed one condition