Add support for account balances

[cameri]

I needed support to grab account balances from a budget and it was not available.

I added support in this PR:

#15

I've also corrected a few typos and added documentation of the feature.

[cameri]

Is this repo still maintained?

[Stuke00]

I would like this feature as well.

[cameri]

@Stuke00 I've implemented this on my own fork which you could use until we hear back from @wxt9861
Here's the link: https://github.com/Cameri/ynab

[wxt9861]

Sorry for the delay. Your PR was merged. Thank you for the contribution.

[cameri]

Thanks, and welcome back :)