Make empty string a valid extension option

[kalefranz]

I have a use case of gulp-swig where I use a .swig extension on the input files...something like template.html.swig. I would like gulp-swig to be able to just remove the extension entirely--i.e. use an empty string for opts.ext. An empty string, however, is falsy, and so this pull request addresses my use case. The current behavior is mostly unchanged, as the default .html extension is still applied if opts.ext is null or undefined.

[colynb]

To be honest, the opts.ext option was put in very early before some gulp best practices. I was thinking of taking out that completely in favor of using gulp-rename for managing changes to the file name/extension. https://www.npmjs.org/package/gulp-rename

[colynb]

by best practices, I mean, plugins shouldn't bake-in things that other plugins are much better at doing... encouraging a more modular eco-system.