For one line embedding allow shortcut `end=start`

[alexander-yevsyukov]

Currently, in order to embed one line of code we specify the same value for start and end parameters of the command. It would be good to allow some kind of shortcut to avoid specifying the same value for the end.

How about lines=1 or size=1?

[alexander-yevsyukov]

Or it could be solved by having only one parameter named say, line, which accepts the same which start does.