Refactor the Pooch class to make it easily subclassed

[leouieda]

From @remram44 comment on #4, it would be great to expose more of the inner workings of the Pooch class as methods. This would make it easy to subclass it and overwrite those methods. The _download_file method should also be made public (remove the leading _).

Any ideas and suggestions on what should be refactored are welcome.

[santisoler]

Would be great to have an easier access to the filenames inside the registry.
Currently, we must search for them inside the registry dictionary, but we must convert the dict_keys to list first. This leads to ugly coding.

For example, if we want to get a list of every filename in the registry we must do something like this:

filenames = list(GOODBOY.registry)

Would be nice to have a more stylish way of doing it, something like GOODBOY.filenames.

[leouieda]

@santisoler OK, that's something we could easily add as a property in Pooch. Maybe registry_files instead of filenames. Would you mind opening a PR for this?

[santisoler]

@leouieda I've just opened #42 for this improvement.

[leouieda]

Thanks! Merged