Feature request: Reading from stdin

```
> cat test-twee/trivial.twee | tweego -f jailbird -
warning: path -: Reading from standard input is unsupported.
error: Starting passage "Start" not found.
```

Passing files via stdin is useful in many situations including as part of toolchains and in docker-based environments. This should be as simple as seeing if `sources` is empty and reading from `os.Stdin` instead.