Line number of errors can be offset by the client.

[epage]

Building on #232, when embedded inside of Cobalt, the liquid will come after the frontmatter, for example

---
title: Foo
---

# {{ page.title }}

Hello World!

As we add line number support, we're going to need to provide a way to handle offsets like this.

Options

1. Pass the offset to the parser.

2. Modify the errors as they come out of liquid.

Passing the offset seems like it'd be the simplest.