Honor file extensions

[ghost]

Check whether Fly honors file extensions if the last filter does not return { extension: ".." }.

For example:

  yield this.source("coffee/src/**/*.coffee")
  .coffee()
  .uglify()
  .target("coffee/dist/")

If uglify does not set the file extension, Fly should use whatever extension set by coffee and so on.

[ghost]

The reduce filter where each of the transformations is applied to the incoming source will use the lastly exported extension. So, in the case above, the result will be .js even if uglify did not specify { ext: ".js" }, because that's fly-coffee exports.