Fix default value of 'number' option in options

Real default value has diverged from default value in options
module.
Prepare data types for 'relativenumber' option
Consider 'relativenumber' on drawing filelist
Use more verbose definition of option handlers

It fits 80 width line length limit and easier to read.
Add 'relativenumber' option
Document 'relativenumber' option

Also highlight it.
Use one variable to control file numbering

Instead of separate variables for 'number' and 'relativenumber'
options.
Merge branch 'add-relativenumber'

Also fix a bug with initialization of 'number' option.
Add 'relativenumber' option to control displaying of file numbers.

Thanks to filterfalse.