Provide a global `only` setting to default to isolated contexts

[benbacardi]

Providing a global setting to default to using isolated contexts for components would be quite useful in preventing unintentional use of parent template context variables inside a component (which is desired behaviour in a lot of cases).

Alternatively this could perhaps be set on a per-component basis inside the component, rather than when the component is called?

I suppose it would also have to come with an equivalent way to negate it for a single component render, i.e. {% bird button inherit %} or something.

[jefftriplett]

It would be great if bird were isolated by default.