Improvements to context variables methods

[oliversong]

Right now it just sets globalOptions.extra to the object you pass in, but it would be nice to be able to add or update a key in globalOptions.extra. It's especially bad because there's no 'getExtraContext' for us to provide an updated object.

Couple ways to go-

• Provide all CRUD methods for ExtraContext?
• Expose globalOptions on Raven object?
• At least make a Raven.getExtraContext method.

This probably applies to Raven.setTagsContext as well.

[oliversong]

Ah I didn't notice this but it looks like someone implemented an "Update" method here - #255. Up to the devs to see whichever they like better.

[henry74]

Is the ability to dynamically change/set the ExtractContext hash available in some fashion or another?

[mattrobenolt]

Closing this in favor of modifications I'm going to make to #255.