Consider a more Unix-y interface for dream_eml

[tmattio]

The current dream_eml CLI takes a FILE argument, infers the output filename from it, and writes this file in the same directory as the input file.

This has been problematic in the past when I wanted to put my eml files in a separate directory. The interface also makes it impossible to create a new dune dialect as the preprocessor is expected to write to stdout.

I think a more Unix-y interface for the dream_eml CLI would make its usage a bit more flexible.

What do you think of allowing it to read from stdin and write to stdout? Should these be the default?