Allow disabling price lookup

[hkjn]

Related to #78.

It would be cool if users could set a flag like --disable-rates to tell spark-wallet to not look up fiat exchange rates at all, thus reducing dependencies on third party servers and potentially improving privacy.

[shesek]

Implemented in 7512aa9, can be set with --no-rates.

Spark will currently show USD amounts as "n/a" if you pick USD as your display unit. Ideally, the client-side should be made aware that exchange rates is off and avoid displaying USD as an option altogether. I created a separate ticket to keep track of that at #82.