Previously we used the following Project Gutenberg conventions
for plain output:
- extra space before and after level 1 and 2 headings
- all-caps for strong emphasis `LIKE THIS`
- underscores surrounding regular emphasis `_like this_`
This commit makes `plain` output plainer. Strong and Emph
inlines are rendered without special formatting. Headings
are also rendered without special formatting, and with only
one blank line following.
To restore the former behavior, use `-t plain+gutenberg`.
API change: Add `Ext_gutenberg` constructor to `Extension`.
See #5741.
