Add format and dry-run flags

• Adds the ability to generate a static asset manifest file in csv format instead of json using the new -format flag
• Adds a way to test your command using the new -dry-run flag which skips uploading to s3

curl -L https://github.com/bufferapp/buffer-static-upload/releases/download/0.2.0/buffer-static-upload-`uname -s` > /usr/local/bin/buffer-static-upload
chmod +x /usr/local/bin/buffer-static-upload