Allow to pass additional options to exiftool

[alexbruy]

It would be very useful to allow pass additional parameters to exiftool. For example, currently if photos contains user-defined or simply unknown tags it is impossible to get this tags via pyexiftool get_metadata() method because this tags only extracted when exiftool is runned with option "-u"

[smarnach]

Valid point. As a hack, you can pass "-u" as one of the filenames to get_metadata_batch(). I'm currently working on the encoding issue you sent by mail; fixing that will also provide a nicer interface for passing custom parameters. Note that the interface will change with these fixes in place, and I will probably make a beta release on PyPI.