Add code to the `settings.query()` method so that, instead of reading
the setting from the URL only, it reads the query setting from the
`js-hypothesis-config` scripts in the host page or, failing that, tries
to read it from the URL.

In `index.js`, after `settings.query()` is called, there are two
`Object.assign()` calls that overwrite values already in the `config`
objects with any returned from `js-hypothesis-config` scripts or the
`window.hypothesisConfig()` function in the host page. So a
`config.query` setting from `settings.query()` already gets overwritten
by a `query` setting in a `js-hypothesis-config` script or
`hypothesisConfig()` function.

This commit makes that overwriting explicit, encapsulates it in the
`query()` method, and unit tests it.

Note that the two `Object.assign()`s are still present in `index.js` so
the `config.query` setting returned by the `query()` method still gets
overwritten by them, but this now has no effect as the `query()` method
will already have set `config.query` to the same value that it will be
overwritten with.

In a future commit the `Object.assign()`s will be removed and there
won't be any overwriting.

Note that **this will change the behaviour of the code** once the
`Object.assign()`s have been removed: the `query()` method reads the
query setting from `js-hypothesis-config` scripts but does not read it
from `window.hypothesisConfig()` any longer. I don't believe it's
necessary to read `query` from `hypothesisConfig()`.