Request: Support for piped YAML

I'd really like if I could pipe a YAML string to the `beats` executable, and have it treat that YAML as the `INPUT_YAML_FILE` if none is specified.

For example, in addition to being able to do this:

```
beats --path="./" tmp/word.beats.txt
```

It'd be great to be able to do something like this:

```
cat tmp/word.beats.txt | beats --path="./"
```
