Display amount in accepted standard

[Prakhar-Agarwal-byte]

Fixes #257 Displays the amount as mentioned in https://medium.com/@mark.nugent.iv/grouping-bitcoins-fractional-digits-an-idea-whose-time-has-come-22d9dad8ac51

Screenshots:

[thunderbiscuit]

Great stuff, thank you for the PR. For the btc denomination I'm looking at them and my favourite is the one where we use small spaces (I can't remember where I've seen this but it was in a related article). Something like this (from the Medium post you linked) but without the dots:

As for the satoshi version, I like how you have it as is.

[thunderbiscuit]

If you can rebase this I think it's ready to merge.