Be more careful with format name #76

XML::Feed::new and XML::Feed::parse build a class name by interpolating their format argument and loading it with a string eval: my $format_class = 'XML::Feed::Format::' . $format; eval "use $format_class". Nothing constrains the name, so a format containing a semicolon has the remainder run as Perl; we confirmed this through both entry points. Both methods document format as an ordinary argument, and choosing a feed format from a query parameter (format=RSS) is a common content-negotiation pattern, so an application can reasonably pass a value it does not fully control.

I suggest the following:

• In the BEGIN block in lib/XML/Feed.pm also create a package variable %formatters which maps format names to a true value
• Whenever we're passed a string to use as a format name, check %formatters for valid values
• die on an invalid value

Also, add tests for this.