More-better-er overrides

Update with long overdue feature to allow omitting the setting of the optional inputs (like Category, Groups and/or Policies) via the *LEAVE_OUT* keyword in the recipe override for a given product.
Put another way, if you want to NOT have the importer check for or optionally create any of those three, use the regular autopkg recipe override process and replace the values for any or all of those keys with (including the asterisks) *LEAVE_OUT*, like so:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Identifier</key>
    <string>local.jss.Firefox</string>
    <key>Input</key>
    <dict>
        <key>CATEGORY</key>
        <string>Barfoo</string>
        <key>LOCALE</key>
        <string>en_US</string>
        <key>NAME</key>
        <string>Firefox</string>
        <key>RELEASE</key>
        <string>latest</string>
        <key>SELFSERVE_POLICY</key>
        <string>*LEAVE_OUT*</string>
        <key>SMART_GROUP</key>
        <string>*LEAVE_OUT*</string>
    </dict>
    <key>ParentRecipe</key>
    <string>com.github.autopkg.jss.Firefox</string>
</dict>
</plist>

As always, please provide feedback in the Issues section here on GitHub or however else you see fit! (Please keep in mind I'm currently only actively testing the most recent JSS release with the products included in my jssRecipes repo, but feel free to let me know if there's other features you need or issues you run in to.) Thanks!

Testing release

Initial release for wider testing. Please share "ideas, patches, documentation, bug reports, or complaints..." (source) by either using issues in this repo, or by contacting me in ##osx-server IRC(Allister) or on twitter, @sacrilicious or my email(shouldn't be too hard to find). Please be so kind as to not report issues in the main autopkg/autopkg repo at this time! Thanks for testing!