Allow undeclared parameters to be defaulted

It would be really useful if you could set any null parameters to a default value. For instance:

snippet.html could contain:

``` html

@@one

@@two

And be included via:
`@@include("snippet.html", {"one":"text"})`

Currently, as nothing is passed in for `@@two` this will will display:

``` plain
text
@@two
```

It would be nice to be able to define a default value for parameters, so any missing parameters can be treated as a blank string by default (or have some default text entered) rather than showing the unused `@@` variable in the output.