Allow the default template file extension to be changed.

apoca on IRC pointed out that it seems impossible to do this; the old
default_tmpl_ext config setting has no effect.

This looks to be the best fix (and will work for all template engines).
Er, config setting should be 'extension'.
Tests for changing template extension
Merge pull request #206 from PerlDancer/dave/change_tmpl_ext

Support changing template file extensions