Feature/use polkadot api

[codespool]

• replace calls to cargo contract in deploy command in favour of polkadot.js api calls
• introduce ChainApi class to handle api interaction
  • also DeployApi subclass with methods specific to deployment
• add listr sequence to deploy command

TODO:

• Utilise instantiate RPC call to get correct gas cost automatically

[codespool]

@hoonsubin
as agreed, I left the api instance in use.
I have added account command with create and list subcommands and the possibility to generate or submit own mnemonic.
Also, the node url is now taken from config.
Some utility functions added as well.

[hoonsubin]

I don't think it's a good idea to log the mnemonic to the console

[codespool]

I was thinking about how it shouldn't be in the in the config neither, that's why there's a warning to only store dev accounts, and this is output only with --verbose flag.
It's useful for example when you want to add those to polkadot.js app to transfer some tokens to it.
That being said, I agree, we can't trust the user not to use those in prod. I'll remove the option for now and think about how to actually prevent using stored accounts for prod deployments.

[hoonsubin]

Lgtm