Include currency value in the `balance` command

[owenvoke]

Detailed description

This is just an enhancement.

Add support for currency checking using the cryptoc balance command. For example:

cryptoc balance -c usd
cryptoc balance -c gbp

Values can be retrieved from a service such as CoinMarketCap. I guess it could get the data, and cache for 5 minutes (as per their note about limits).

Context

This would be useful to allow viewing of your local currency value, rather than just the token value.

Your environment

• PHP 7.1
• Ubuntu 17.10 / Windows 10

[owenvoke]

Added in a feature branch. I need to fully test it first.
feature/balance-currency-value

[owenvoke]

I'll add further tests later.