Add code to the `settings.annotations()` method so that, instead of
reading the setting from the URL only, it reads the annotations 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.annotations()` 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.annotations` setting from `settings.annotations()` already gets
overwritten by a `annotations` setting in a `js-hypothesis-config`
script or `hypothesisConfig()` function.

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

Note that the two `Object.assign()`s are still present in `index.js` so
the `config.annotations` setting returned by the `annotations()` method
still gets overwritten by them, but this now has no effect as the
`annotations()` method will already have set `config.annotations` 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 `annotations()` method reads
the annotations 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 `annotations` from `hypothesisConfig()`.