A different plan for the top-level word_frequency function.

When, before, I was importing wordfreq.query at the top level, this
created a dependency loop when installing wordfreq.

The new top-level __init__.py provides just a `word_frequency` function,
which imports the real function as needed and calls it. This should
avoid the dependency loop, at the cost of making
`wordfreq.word_frequency` slightly less efficient than
`wordfreq.query.word_frequency`.