Add Markdown filter

[racke]

We are using the following Template::Flute filter in the Perl Dancer Conference website:

https://github.com/interchange/Perl-Dancer-Conference/blob/master/lib/PerlDance/Filter/Markdown.pm

So in order to integrate that with Template::Flute, we need to do the following things:

• Change module name
• Write tests which are only running when prerequisites are satisfied
• Adjust POD to match other filters
• Add to main POD

The existing currency filter can be used as example.

[SysPete]

We need to allow options to be passed through to HTML::Scrubber for allow, deny and rules. We could perhaps have '-allow/+allow' to remove/add options and 'allow' to replace? Rules will need to be handled slightly differently since it is a hash.

[racke]

Yes, that would be a good idea. But let me get it in just as-is before thinking about the options, so I can use it in the demo shop.

[racke]

Added through 8f9e7180132f0ea22c7255e32e7decb161641d3e.