Multiple output formats

[strarsis]

The picture element can be used together with srcset to provide multiple image formats (notably webp + png as fallback), it would be great if gulp-responsive could output the same image in multiple formats.

The format option expects a string, for multiple formats, it would make sense letting it expect an array. It is possible to have multiple output formats with current gulp-responsive, but this requires redundant configuration.

[rejas]

Would be really awesome to do this, any ideas on how to do this @mahnunchik or @strarsis ?

[mahnunchik]

PR is welcome

[rejas]

@mahnunchik PR is ready :-)