Support all as a placeholder for full balance

[canselcik]

This is a feature I always thought would be nice to have as a part of the add command and would be great for iadd too.

Essentially the ability to say all and have that be substituted with the current balance of an account.

Let me know what you think. It should be a pretty small change.

[hpdeifel]

Seems reasonable if there's a need for it, but tbh I've personally never needed it.

How often do you dump your whole account into one transaction? (I.e. Please give a real world example, so that I can better understand what the use case would be)

[canselcik]

I use it to keep track of my crypto-currency transactions. I often transfer a certain amount of funds into an exchange and trade all of it for another currency. In fact probably 75% of my transactions are such transactions.