Streamline goal grouping/splitting/combining tx files

## Is your feature request related to a problem? Please describe.
It is awkward to concat separate transaction files to assign group indicies, then split them, sign them, then recombine them.

## Describe the solution you'd like.

A modification to the `goal clerk` subcommands to allow a more straight forward workflow.

## Additional context.

It is even more awkward when running goal in the sandbox, as an attempt to make it easier I tried adding a generic exec command to the sandbox but that wasn't a great solution.
https://github.com/algorand/sandbox/pull/32

If `goal clerk group` was capable of accepting multiple tx files as input, you'd be able to skip the `concat` step. Second option might be another command like `goal clerk split` but combine or something.

Another thing to consider is often the tx filenames will be meaningful to the creator so combining `asa_xfer.tx` and `algo_payment.tx` into `grouped.tx` then splitting them to `split-1.tx` and `split-2.tx` is not ideal.

I can submit a PR to address this but I'd like some guidance.

Currently thinking `goal clerk group -i file1.tx file2.tx ` where if no `-o` is specified it assigns group indicies to the transactions in the files in the order they're given. Then signing can be performed on the meaningful filenames. Finally they can be concatenated (using `cat` or new subcommand `goal clerk combine`?) for the final grouped and signed tx file and sent.