Alternate formatting of translation string interpolation

[grabner]

hi pylons team,

i much prefer python's string.format() string templating rules than the templating provided by translationstring.TranslationString.interpolate(), and have monkey-patched translationstring to use that instead for my projects.

i'd like to make that a bit less hackish though: if i forked translationstring to accept a new parameter "formatter" to TranslationString and TranslationStringFactory (defaulting to the current formatting rules to stay backwards compatible), would you accept the patch?

cheers