Data URI encoding of named files

Here is a small extension that encodes the named file as a data URI and inserts it in to the file you are editing. This is mostly useful when creating standalone CSS/HTML documents, and embedding images and other external resources.

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This extension does not fit the pattern of the other functions provided by unimpaired, so I understand it may not be an appropriate fit. It is differs in the following ways:

* It relies on external programs to do most of the work - `mimetype` and `base64`. These are not guaranteed to be installed on all systems.
* It only has an encode option, not a decode option. There is no way of recovering the filename from the encoded file, so this is a one way operation.

If this is not a good fit for `unimpaired`, I am happy to discuss ways I can change it to make it fit.