Unable to use FreeMarker template tags as juice.codeBlocks

**Issue**
I'm using FreeMarker templating tags inside an email template. However, when I try to exclude these using the `juice.codeBlocks` global, it still parses them.

```
juice.codeBlocks = {
start: '<#',
end: '>'
};

juice('<#list myObject.values as value>');
```

**Expected result**
`<#list myObject.values as value>`

**Actual result**
`<#list myObject.values="" as="" value="">`

**Example**
https://runkit.com/587926812ee9040013e6cc04/587926812ee9040013e6cc05

Start with the linked Runkit reproduction and trace how juice.codeBlocks is handled when juice() processes the FreeMarker template tags. Confirm the fix by checking that the tags remain unchanged rather than being parsed into HTML attributes.

Written by the indexing model from the issue text.