Custom readers and writer paths

[John MacFarlane <jgm-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org>]

I believe that currently one must specify the path to a custom
Lua reader or writer: you can't put it in your data directory,
as you can with a Lua filter, and it won't be found in your
executable PATH.

(Is that correct, Albert?)

My question is whether we should change that.  I think it could
be quite convenient to allow custom readers and writers to be
distributed as Lua rocks.  You could then do `luarocks install
cool-pandoc-writer` and it would put `cool-writer.lua` in the
`bin` directory of your luarocks installation. Pandoc could then
be trained to look in the executable path for a custom writer
if it is not found locally.

Alternatively, we could search a custom-writers and custom-readers
subdirectory of the user data directory (as we do with filters).

It is a little cumbersome to have to copy the default
writer/reader somewhere and pass its path to pandoc.  This may
be good from a security point of view, however, as it makes it
less likely that people will blindly use custom readers/writers
from third parties, not realizing that a custom reader/writer
could in principle do just about anything on your file system.

A related question: would it be possible for pandoc to set up
the Lua environment in which custom readers/writers are run so
that it is "sandboxed," limiting I/O operations to (e.g.) a
local directory, or perhaps logging I/O operations as pandoc
warnings?

Thoughts welcome.

John