support symlink field in 'files'

```
                  {
                      "id": "bootplate-moonstone",
                      "type": "template",
                      "version": "0.2.3",
                      "isDefault": true,
                      "description": "Enyo moonstone app",
                      "class": "moonstone",
                      "deps": ["enyoappinfo", "enyoicon"],
                      "files": [{
                          "url": "@PLUGINDIR@/templates/app-moonstone",
                          "symlink": {
                              "enyo": "@PLUGINDIR@/templates/bootplate-moonstone/enyo",
                              "lib": "@PLUGINDIR@/templates/bootplate-moonstone/lib"
                          }
                      }]
                  }
```
* symlink intends to make symbolic links for local directories or files

Enyo-DCO-1.1-Signed-off-by Junil Kim <logyourself@gmail.com>