Allow setting dictionary and list values

The following are valid statements:
```
<esi:assign name="streets" value="[ 'Brannan', 'Bryant', 'Howard' ]"/>
<esi:assign name="streets{2}" value="Folsom"/>
```

```
<esi:assign name="cookies" value="{ 'chocolate chip' : 10, 'oatmeal' : 15, 'sugar' : 10 }"/>
<esi:assign name="cookies{'sugar'}" value="9"/>
```