Support / instead of : syntax

[cyruseuros]

This feels like a remnant of ledger/hledger/beancount, especially as the colon is part of yaml syntax and can get visually confusing. / clearly denotes belonging/sub-accounts, while : is only intuitive for lispers (and possibly C++/Rust devs given ::).

For those choosing transity as their first plain text accounting tool, I feel like this is a simple change to make things a little more intuitive, while the : could be kept around for those coming from other plain text accounting tools.

[cyruseuros]

Support for nested accounts would also be a nice touch. Right now john:wallet and john:visa gives me balances for john, john:visa, john:wallet. One the other hand john:bank1:visa, and john:bank1:master don't produce any balance for john:bank1