Make HTML export work.

Make HTML export work.

The key is the new copier script lib/scripts/ext_copy.py. Usage:
 ext_copy.py [-e ext1,ext2,...] <from file> <to file>
Given a <from file> and <to file>, it will copy all files in the directory
in which from_file is found that have the extensions given in the -e option,
or all files in that directory if no such argument is given. So, for example,
we can do:
  python ext_copy.py -e png,html,css /path/from/file.html /path/to/file.html
and all html, png, and css files in /path/from/ will be copied to the
(possibly new) directory /path/to/file.html.LyXconv/. The -t option determines
the extension added to the output filename passed to the script ($$o, in the
copier definition), the default being "LyXconv". If just . is given, then no
extension is added.

Other changes:
* configure.py: added appropriate copier definitions for html and wordhtml
formats, as well as for the Program pseudo-format.
* lib/Makefile.am and development/scons/scons_manifest.py: add ext_copy.py.
* lib/doc/Customization.lyx: update documentation.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18791 a592a061-630c-0410-9148-cb99ea01b6c8