rose env-cat --match-mode=brace

Modify `rose env-cat` to support a `--match-mode=brace` option.

* In `--match-mode=brace`, the command will only consider `${NAME}` syntax for environment variable substitution.
* New `--output=FILE` option to specify an output file other than `STDOUT`.
* Simplified regular expression to fully capture the symbol to substitute.
* Pylint score enhancements.
* Test borrowed from #1250, but modified.

Supersede #1250. (I believe this user interface should leave us more room to expand the command in the future.)

Close #1246.