Accept input from STDIN as well as a file

I would like to be able to do `pbpaste | html2slim | pbcopy` to process the contents of the clipboard.

Current usage:

```
Usage: html2slim INPUT_FILENAME_OR_DIRECTORY [OUTPUT_FILENAME_OR_DIRECTORY] [options]
```

Desired usage:

```
Usage: html2slim [INPUT_FILENAME_OR_DIRECTORY [OUTPUT_FILENAME_OR_DIRECTORY]] [options]
```

`erb2slim` should be changed similarly, of course.
