Support using persistent user configuration for most flag functionality

[shonfeder]

This is generally desirable. E.g., I don't have the default Sacramento font installed, so I have to remember to add a --font flag every time I sign. I'd like to set this once somewhere.

A lightweight approach that a lot of CLI tools I use adopt is to make make every flag option also something you can set via an envvar. E.g., I'd love to be able to just drop THEMIS_FONT=foo into my bashrc.

A config file located in the .themis/contract dir would also be OK.

[thanethomson]

I'll look at integrating Viper support here for Themis Contract configuration.