resolves #57 update table syntax in quick and writers docs

- added table example with column titles on single line to quick ref
- added table example with cols and repeat operator to quick ref
- clarified column relative width in quick ref
- added repeat operator definition to writers guide
- updated cols value syntax in quick and writers ref tables

Also related to issue [386](https://github.com/asciidoctor/asciidoctor/issues/386).