Support the -b flag for specific doc type

[xcoulon]

so we can have -b manpage as an alternative to specifying the doctype in the document itself, with

:doctype: manpage

[gdamore]

Actually -b is for the backend, -d is for the doc type.

I'm fixing the support for -b as part of the fix for #601

[gdamore]

(And for the curious, yes, manpage is a possible backend for asciidoctor -- it is distinct from the doctype manpage. The backend manpage emits TROFF sequences. The doctype man page affects the rendering of the title section.)

[xcoulon]

Actually -b is for the backend, -d is for the doc type.

ah right 🤦‍♂️

I'm fixing the support for -b as part of the fix for #601

I know it's a minor change but it could be addressed in a separate PR (easier to track the change afterwards)

[gdamore]

It seems kind of linked -- impossible for anyone to access the XHTML output (short of writing a program to do so) otherwise.

[xcoulon]

well, can't you fix this issue here first, and then use it for the XHTML output issue?

[gdamore]

I suppose I could have, but I did the development in the other direction. I'll submit the unified PR. If it bothers you greatly, it doesn't look too bad to split apart.

[gdamore]

This was fixed with the merge of #618 .