Option to select shortest item in the duplicate list for process.dedupe

[smtodd]

When using thefuzz.process.dedupe to deduplicate lists and assign a single name to an entity, the only choice so far has been to select the longest item in the group. This PR allows a choice between using the longest name or the shortest name. It makes the following changes:

1. Add an optional parameter "len_selector" to process.dedupe. Possible values are "longest" and "shortest". "longest" is the default and maintains current behavior. "shortest" will select the shortest item in the list of duplicates. This is useful in cases where the shortest item has the most generalized information about the entity, and could thus include the others. An example could be a department in a university.
2. Defines behavior for both "longest" and "shortest" parameter values.
3. Adds information about this parameter to the docstring