-p, --sheetdelimiter help and code incorrect

[BrianInglis]

xlsx2csv --help shows:

  -p SHEETDELIMITER, --sheetdelimiter SHEETDELIMITER
                        sheet delimiter used to separate sheets,
                        pass '' if you do not need delimiter, or 
                        'x07' or '\f' for form feed (default: '--------')

but '\f' is '\x0c' not '\x07' and the argument is passed to python chr(int(sheetdelimiter[1:])) which converts it to '\a' not '\f'.
Either the sheet delimiter would have to be passed as 'x12' which gives '\f', but just looks so wrong, or change the code to specify the base as hex chr(int(sheetdelimiter[1:],16)) if the prefix is 'x', so then 'x0c' can be passed, and the documentation updated to match. Other approaches might also be reasonable.