Added AsciiDoc backend

[declantsien]

See Convert documents from AsciiDoc to Org . Need some extra effort to work.
Might be a good idea to include this.

[tecosaur]

At a glance this looks reasonable, you'll want to add org-pandoc-export-to-asciidoc here though:

org-pandoc-import/org-pandoc-import-transient.el

Lines 37 to 39 in d3cd2b0

	(when (featurep 'ox-pandoc)
	'(("rst" . org-pandoc-export-to-rst)
	("docx" . org-pandoc-export-to-docx))))

[declantsien]

I wouldn't try to edit asciidoc source files from the Internets. It's properly result in some formatting changes.
I use this primarily to read files in Org mode, since I am more comfortable with it.

But still it's a valid use case for asciidoc files which are produced using ox-pandoc though. Made the change.

Also org-pandoc-import-transient-mode is buggy. Reported it here #16

[tecosaur]

Thanks for that.

Looking at the updated PR, I'm a tad wary though about .txt being dedicated as an asciidoc extension, since its use is far more general.

[declantsien]

I added it cause it's used by ox-pandoc and org-asciidoc. I wouldn't worry too much about it.

Currently there is only on backend associated with txt. Later if there are mroe backends provide txt, maybe we can add a prompt for org-pandoc-import-[as,to]-org to choose which backend to use?