[options-] disable adding rows

[midichef]

Running add-row on the OptionsSheet produces an error:

  File "/home/midichef/.local/lib/python3.8/site-packages/visidata/basesheet.py", line 200, in execCommand
    escaped = super().execCommand2(cmd, vdglobals=vdglobals)
  File "/home/midichef/.local/lib/python3.8/site-packages/visidata/basesheet.py", line 73, in execCommand2
    exec(code, vdglobals, LazyChainMap(vd, self))
  File "add-row", line 1, in <module>
  File "/home/midichef/.local/lib/python3.8/site-packages/visidata/sheets.py", line 212, in newRow
    return type(self)._rowtype()
TypeError: __init__() missing 2 required positional arguments: 'name' and 'value'

This commit disables add-row for OptionsSheet.

I also explored adding an empty row by making OptionsSheet.newRow():

def newRow(self):
    return vd.option(None, None, None, None)

but I was not able to edit the name field of the resulting row, only the value.

[saulpw]

Makes sense to me. BTW, vd.fail raises an exception, so the return is unreachable. Also we don't need an empty return in Python anyway (some people prefer to be explicit, but I prefer to elide it).