Add ability to assign value to hash

[mikebridge]

{% assign myhash.newvalue = 1 %} should add/update a value in a hash. Similarly,{% assign myhash.mysubhash.newvalue = 1 %} should work as expected.

Assigning to a non-existent value or a non-hash should generate an error.

This should work for capture too.

(Note: This is not a feature of Shopify Liquid)