Needless inflexibility

This plugin can apparently only be used to output `.swift` files from `.gyb` files.  There's no reason you couldn't use gyb to generate HTML, C++ (the Swift project used to do that), or any other language you like ; that's why all the files in the swift project are suffixed with `.swift.gyb`: the idea is that the name of the file generated by the tool should be the input file name, sans extension.