Add option to suppress numbering of separator lines between files

[delphidabbler]

At the moment lines separating concatenated files are numbered.

It would be useful to have a Boolean command, say --separator-line-numbering that enables or disables line numbering.

The command would only take effect if line numbering was enabled - i.e. it would not override the --line-numbering command.

[delphidabbler]

This issue has been split out of issues #33 and #48.

[delphidabbler]

If option to add file name at top of files is implemented (see issue #33) then this will have an effect on how --separator-line-numbering works.

Do separator lines end before file name? If so then there's no problem. But if separator lines contain file name then --separator-line-numbering may need to take a pair of Boolean parameters - say on/on.

[delphidabbler]

If we arrange for the 2nd parameter to be ignored if file names are not being output then the default value can be on/on without causing any problems.