Add .marsrc.json config file handling with multisig section loaded as extension

[marcin-trust]

I want to store frequently used mars configuration in a file. Especially when there's a need for complex structure for some extension feature (e.g. multisig)

{
    "multisig": {
         "gnosis": {
             "safes": {
                 "rinkeby": "0x......"
                 "mainnet": "0x....."
           }
           "etc.": ""
         }
     }
}