Markdown.pm: make Markdown module available to use

Markdown.pm: make Markdown module available to use

In order for the "Markdown" module to be available to "use", this
must succeed:

    use Markdown;

But it must succeed in two different environments:

 1) the pre-install environment
 2) the post-install environment

In the first case, that means a symbolic link into the checked out
markdown.git project.  In the second case, that means installing a
symbolic link into $Girocco::Config::basedir that points to the
installed copy of Markdown.pl.

Since "use" statements are processed even when just using the `perl
-c` command to check syntax, failing to address the first case would
make it very difficult to check syntax prior to installation or use
any module during installation that referenced the Markdown module
even if the Markdown functions were not actually being called.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>