ASCII Export: Implement export to buffer

ASCII Export: Implement export to buffer

Samuel Wales writes:

> I frequently export to ascii without wanting a file to be created,
> especially not in a useful directory, as the files are temporary.
>
> Is there a way to export ascii to just a buffer?

There is now, `C-c C-e A'.

This commit also implements commands
- org-export-as-ascii-to-buffer
- org-replace-region-by-ascii
- org-export-region-as-ascii

which are similar to what is available for HTML and LaTeX.

`C-c C-e A' used to be the key for publishing all projects.
This functionality has now been moved to `C-c C-e E'.