Allow "on" and "off" as boolean values

[philrandal]

Sometimes when using ini files the states of "on" and "off" make more sense than yes/no or true/false

Pull request #154

[lmoellendorf]

@philrandal
Thank you for your contribution. I like the idea to implement this. As it changes the way existing INI files are parsed this will have to wait for a post v4 release.