Express functionality with operators and movements/text objects

[jjaderberg]

Create one or more opfunc functions that conjure AsciiDoc. Not sure if all operations can be expressed this way, possibly they can. Any progress along this route will in any case make the plugin much more powerful and much less verbose.

In connection, express interesting chunks as objects or motions. This is done already for sections, but there is a large potential for improvement here.

This is a long running target. Here are some attractive operations to wet the appetite:

• Turn motion from cursor to end of next paragraph into their own section. Section title added at depth parallel to current section, with cursor in insert mode ready to reduce/increase depth and add a title.
• Turn a listing block into a verse block.
• Turn motion from cursor backward two blocks into an admonition block.
• Turn a sentence into a block of some kind (while editing someone else's source, obviously, because we keep one sentence per line so this is already possible).