Hello,

The following patch implements a draft mode for export. When in draft
mode, invalid macros and links do not throw an error. It can be toggled
with `org-export-as-draft' variable, or using C-d in export dispatch.

It introduces a backward incompatible change since it modifies signature
from `org-export-as' and alike.

This patch is incomplete as it is missing some documentation, an entry
in ORG-NEWS and some tests. Also, export back-end in contrib/ directory
are not updated yet. In any case, feedback welcome.

It is meant to be applied on top of master branch.


Regards,

-- 
Nicolas Goaziou                                                0x80A93738