Implemented auto-escape functionality and provided a default escape replacer for HTML.

[dop251]

This pull request adds auto-escape functionality which is similar to implicitly applying a filter to the output of an expression block, but done in a slightly efficient way (using a mechanism similar to strings.Replacer which allows for streaming processing without excessive memory allocations).

I know this functionality is not part of liquid (there is an open issue which doesn't have any responses at the time of writing), but I think this is an extremely useful feature and it does not break any existing functionality.

Checklist

• I have read the contribution guidelines.
• make test passes.
• make lint passes.
• New and changed code is covered by tests.
• Performance improvements include benchmarks.
• Changes match the documented (not just the implemented) behavior of Shopify.