Added config support for specifying the output file prefix

This was hardcoded to "app-" because that's what I wanted mine to use. No real reason it has to be that way, though. Only offering a prefix is still pretty limiting, but better than hardcoding.

config.go

@@ -8,4 +8,6 @@ type Config struct {
 	Minify bool
 	// OutputDir if specified will cause the result to be written to this directory
 	OutputDir string
+	// FilePrefix is a string that will be prefixed on the files if OutputDir is specified
+	FilePrefix string
 }