Feature request: Subs command with delimiter other than slash

[Kayzels]

The default Neovim (and Vim) :substitute command allow delimiters other than / to be used, and can be any single-byte character except for an alphanumeric character, \, ", or |. I tend to use commas, as there have been patterns I want to replace that contain /. So, I'd actually forgotten that the default is to use /.

When using the Subs command, I was getting unexpected errors when trying to replace a pattern, and it seemed that the functions were receiving empty strings. I only realized it was because I was using commas instead of slashes.