Apply bold formatting to cells in table header row in manpage output

[mojavelinux]

Cells in the table header row are typically stylized using bold formatting. This is true of many man pages that contain tables. The manpage converter is not currently applying this style. As a precedence, this formatting is applied by the manpage output of the DocBook toolchain, so it's reasonable to expect that it is supported by the converter in Asciidoctor as well.