Add support for parsing stdin in CLI

[mgradowski]

Hey! The CLI is missing a way to read files or output from other tools, which is essential for my use case (statements with hundreds of LOC). My contribution is a pretty low effort way to add it.

I decided to use - as a sentinel value [1] to indicate that the CLI should parse the stdin instead of argv for two reasons:

1. It's an established convention in many of GNU coreutil tools [2].
2. I don't think there's a dialect for which - would parse successfully.

[1] https://en.wikipedia.org/wiki/Sentinel_value
[2] https://www.gnu.org/software/coreutils/manual/coreutils.html#Common-options

[tobymao]

thanks