Tpac: Support for skinning via Apache configuration

Tpac: Support for skinning via Apache configuration

More generally, support for augmenting the Template Toolkit template
path list to allow for per-vhost template paths.

For example, a 3-tier consortium could do something like this:

<VirtualHost ...>
    [...]
    PerlAddVar OILSTemplatePath "/openils/var/templates_branch_abc"
    PerlAddVar OILSTemplatePath "/openils/var/templates_system_xyz"
    PerlAddVar OILSTemplatePath "/openils/var/templates_shared"
</VirtualHost>

Signed-off-by: Bill Erickson <berick@esilibrary.com>