add osName to weex.config.env

Since the weex.config.env.platform on web has tell user that the environment is 'Web', there should be another property to tell which system ('android' or 'iOS') it is on. So a weex.config.env.osName is needed. On Android and iOS platform, the platform name and the osName is the same, however on Web platform (webview or mobile browsers), they are different.

For example, a weex page runing in a chrome browser on a Android device with weex-vue-render. access the two properties will be:

• weex.config.env.platform: 'Web'
• weex.config.env.osName: 'android'
• weex.config.env.osName: 'iOS'

doc: http://weex.apache.org/references/weex-variable.html