Add "dest" param for compiler.convertPo in order to check the extension of the destination file

[LiviuNicolae]

This change is related to the modifications i made on the angular-gettext-tools project in order to support c# file generation. So you can add in your Gruntfile.js in the nggettext_compile task another destination file having .cs extension (ex. nggettext_compile: { all: { options: { module: 'app.core', namespace:'test', defaultLanguage:'fr' }, files: { 'Source/WebUI/Scripts/app/core/translation/translations.js': ['po/.po'], 'Source/Core/Common/Translations.cs':['po/.po'] } }, },)