org-refile: Optionally prefix refile targets with buffer name

org-refile: Optionally prefix refile targets with buffer name

* org.el (org-refile-get-targets): Add case to optionally prefix
refile targets with the buffer name.
(org-refile-use-outline-path): Add new option setting and document.

Having an option to use the buffer name as a prefix is convenient,
since this will work hand in hand with uniquify to only show those
parts of the filesystem path needed to disambiguate buffers of
identically named files, as opposed to prefixing refile targets with
the full filesystem path.