Make OGRe invocable directly and implement paging. (4.1.0)

Many updates have been made in this release. First, the retriever can
be called from the command line now using `python -m` or (if installed
via pip) `ogre`. Either method supports long arguments that align with
a newly unified interface among the main entry point (ogre.fetch),
source modules (e.g. ogre.Twitter), and the new validation module.
Moving parameter validation to a separate module allows for more
isolated tests and should improve the speed of validation as parameters
are now checked once, in a uniform manner, by all source modules. All
sources from here out should have an additional sanitation function
that invokes the validation module and prepares parameters for the API
they are responsible for accessing. Finally, when a query is made with
Twitter’s API, there is no guarantee that all the results will be
geotagged. This meant that a query requesting 100 results may cause
OGRe to filter out 90+ of them due to missing location information. Now
the retriever will request more results (pages) until it satisfies the
desired quantity to the best of its ability. Documentation has been
updated to reflect these changes.