Support accessing object value with a variable key in syntax

[xizhao]

Being able to support something like:

{{#each someArray}}
{{../otherObject.[this]}}
{{/each}}

is such a basic feature to tempting engines, especially for any application that relies heavily on unordered or key-value oriented data. It is possible to see this in the future implemented natively? Writing a helper for something so fundamental seems a bit out of place.

[kpdecker]

Tracked under #569