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

[astrofrog]

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

[astrofrog]

@mdboom - is a CHANGELOG entry needed?

[mdboom]

As a new feature, I would mention this briefly in whatsnew.

[tacaswell]

Yes, and definitely api_changes.rst and maybe a whats_new.rst

[astrofrog]

@mdboom @tacaswell - Does this go in the New plotting features section in what's new? Since this doesn't break the current API, is an entry in api_changes.rst needed anyway?

Also, is :context: True valid or do I not need to worry about this?

[mdboom]

I don't think we need an api changes entry -- that's really only for backward-incompatible things.

I would put it in a new section in what's new called "Sphinx extensions" (which we don't normally have, because it doesn't change much).

I don't think context True and False need to work. The old docs saying "bool" essentially means "present or not".

[astrofrog]

@mdboom - done - is this ok?

[tacaswell]

@mdboom interesting. I apparently miss-understood that.

[astrofrog]

@mdboom @tacaswell - the Travis error is unrelated.

[astrofrog]

@mdboom @tacaswell - does this look good to go?

[tacaswell]

@astrofrog This needs a rebase. I suspect from conflicts from your other PR

[astrofrog]

@tacaswell - thanks for letting me know - I've now rebased this

[astrofrog]

@tacaswell - thanks for merging this!