doc/asciidoc: allow documents to pass a global asciidoc configuration

Currently, a document can specify an output-specific configuration, like
the text output, that hides images references, and formats hyperlinks.

But sometimes it is required that a specific configuration is used for
all the output formats, to always apply the same markup rules in all
documents (like using the [specialwords] section.)

Also look for a file named 'asciidoc.conf' in the document's directory.
If that file exists, add it to the asciidoc options (before the
output-specific config, so the latter takes precedence over the former.)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>