[FileFormats] add option for generic names

[odow]

Closes #1617

[nickrobinson251]

Thank you for adding this! I was just about to post a question on Discourse asking how to write a JuMP Model to the .rew format (https://www.gurobi.com/documentation/9.5/refman/rew_format.html), and this looks to fulfil that need.

Is it perhaps worth mentioning .rew in the docstring here, or somewhere in the docs, in order to make this new generic_names functionality discoverable by people looking for that format?

[odow]

Added. From JuMP, you'll be able to go write_to_file(model, "model.rew").

[odow]

The names are going to be slightly different than Gurobi, because we don't include the C1(nnz) in the column, but it's anonymous, which is what the main benefit is.