Allow :context: directive to take 'reset' option. Fixes #2892.

This allows:

```
:context: reset
```

and resets the plotting context. This is crucial when if one wants to use ``:context:`` for multiple figures in documentation. See #2892.

One question - should:

```
:context: True
```

and

```
:context: False
```

work? If so, I'll need to slightly adjust ``_option_context``.

cc @mdboom