Parse expressions in Liquid::Variable#parse.
flat_map
Merge pull request #462 from Shopify/flat_map

nodelist flat_map over map.flatten
Merge pull request #443 from Shopify/completely-parse-variables

Parse expressions in Liquid::Variable#parse.
Separate ? and - into special tokens
Don't allow - to end a variable name
Merge pull request #463 from Shopify/stricter-identifiers

Separate ? and - into special tokens
Avoid parallel assignments