Feature request: Supplying comment text from stdin (or file)

It would be useful to be able to pass the text for a `comment` from stdin, or a file.

Example:
```sh
echo "Test" | basecamp comment 123
cat /tmp/comment.md | basecamp comment 123
basecamp comment 123 --from-file /tmp/comment.md
```

Arguably, the file argument is not needed if stdin is supported.

I imagine other commands would benefit from such improvement as well.
