Document Dumper methods available for use

[toddr]

As mentioned in #89 , We should document the Dumper methods available for use.

looking at the code, Template::Plugin::Dumper currently allows the
following arguments to be passed through to Data::Dumper : Indent Pad
Varname Purity Useqq Terse Freezer Toaster Deepcopy Quotekeys Bless Maxdepth

Adding Sortkeys to this list to order hash contents consistently would
be helpful.

(also only Pad, Indent and Varname are documented in POD)

[petdance]

See also #103