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?

[aantron]

I agree with this in principle and see the advantages. However, eml needs a FILE argument to generate locations in its output that refer to the original source file, i.e. #22 src/foo/bar.eml. That's why the command line is not Unix-y to begin with. Do you really need reading from STDIN, or just writing to STDOUT, like in @tcoopman's PR? An interface where eml always reads from STDIN would be a bit awkward, since it would look like dream_eml FILE.eml < FILE.eml > FILE.ml.