Bug in REPL when defining "input" document

Example:

```
torin:~$ docker run -it --rm openpolicyagent/opa:0.4.0
OPA 0.4.0 (commit 4ea2b27, built at 2017-01-25T18:34:56Z)

Run 'help' to see a list of commands.

> input = 1
error: 1:1: missing input document
```

The problem is that the statement (`input = 1`) is compiled as a query before checking if it's parse-able as a rule. The compile step fails and so an error is printed.