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.

compile.go

@@ -104,7 +104,7 @@ func finalize(config Config, buf map[FileType]*bytes.Buffer) (map[FileType]*Comp
 					return nil, err
 				}
 
-				destFile := filepath.Join(dir, "app-"+ret[key].Hash+ext)
+				destFile := filepath.Join(dir, config.FilePrefix+ret[key].Hash+ext)
 				if err := ioutil.WriteFile(destFile, bytes, 0644); err != nil {
 					return nil, err
 				}

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
 }