Feature/default config

[free-bots]

this pr includes changes of #633 and allowes to share a configuration across multiple clients, if no configuration was provided localstorage will be used like before
open points:

• document behavior if config.json is present in deployment directory
• add a warning in documentation about security risks with a public accessible secret
  related issues: Add a json config file for AriaNg's (own) defaut settings #567, Permanent "AriaNg Settings" #598,

[mayswind]

I'm afraid I cannot merge this pr, because it is not secure when aquiring data which may have sensitive config. So AriaNg does not provide this method to reduce careless behavior. In the other hand, AriaNg provides command api to allow people to set rpc server conveniently and clearly.
By the way, #633 is great, and I would merge it before the next release of AriaNg.