allow $context->get() to get full context

[DaveMeade]

This change updates the Context.php get() method to return the full context if the $variableName argument is null. This can be useful in custom helpers.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @DaveMeade to sign the Salesforce Inc. Contributor License Agreement.

[DaveMeade]

I've signed the CLA, but the process gave errors. Trying again it tells me I've already signed the CLA.