text: Title: [tsv] add CLI option to use NUL as delimiter
  Body: It's useful to parse output from GNU grep's `-Z` option. That produces lines that in Python are `f'{filename}\0{line}\n'`, instead of the usual `f'{filename}:{line}\n'`.
  
  Right now the command line can't be used to specify a NUL delimiter, as in `vd --delimiter="\0"`, because `sys.argv` strings are NUL-terminated and can't ever contain NUL.
  
  My workarounds for now are to use .visidatarc, either add a temporary line:
  `vd.option('delimiter', '\x00', 'field delimiter to use for tsv/usv filetype', replay=True)`.
  or add a new filetype to allow `vd -f nsv`:
  ```
  @VisiData.api
  def open_nsv(vd, p):
      tsv = TsvSheet(p.base_stem, source=p)
      tsv.delimiter = '\x00'
      tsv.reload()
      return tsv
  ```
  Can `open_nsv()` be written without `reload()` right now? I couldn't think of another way to set `delimiter` for TsvSheet.