add usamin support

exe/mikunyan-image

@@ -2,7 +2,12 @@
 require 'mikunyan'
 require 'mikunyan/decoders'
 require 'fileutils'
-require 'json'
+begin
+    require 'usamin'
+    require 'usamin/overwrite'
+rescue LoadError
+    require 'json'
+end
 
 opts = {:as_asset => false, :outputdir => nil, :sprite => false, :pretty => false}
 arg = nil

exe/mikunyan-json

@@ -74,7 +74,12 @@ if opts[:yaml]
     require 'yaml'
     puts YAML.dump(assets)
 else
-    require 'json'
+    begin
+        require 'usamin'
+        require 'usamin/overwrite'
+    rescue LoadError
+        require 'json'
+    end
     assets = assets.map{|k, v| [k, obj64(v)]}.to_h
     if opts[:pretty]
         puts JSON.pretty_generate(assets)