Ability to include "literal" shell variables in the system-environment action

After installing Sun-Java on a Red Hat server, I wanted to ensure that the just-installed version of Java was the one visible in the path. So I expected to be able to use 0.7.X's `pallet.crate.environment/system-environment` to create a file named "java.sh" with the content:
```sh
export JAVA_HOME=/usr/java/jdk1.6.0_33
export PATH=$JAVA_HOME:$PATH
```

However, when I attempted this, I was unable to keep Pallet from looking up the value of the environmental variables. So it substituted those values into the text of the file instead. It did this regardless of \\how \\\\much \\\\\\\\escaping \\\I \tried.

There was a simple workaround to just use `remote-file` with the `:local-file` and `:literal` keyword arguments. I'd like to have _something_ like the following work:
```clj
(system-environment (array-map :JAVA_HOME "/usr/java/jdk1.6.0_33", :PATH "$JAVA_HOME:$PATH") 
                    :literal true)
```

# TLDR; I CAN HAZ `:literal` OPSHUN IN `system-environment` PLS?