Rework grouping logic to index by day, month or yr

- rename env var to RUST_LEDGER_FILE
- update dependencies
- README updates

Parse and accept all ISO-4217 currencies

Refactor CSV module

- Refine account matching logic (no-op)
- Handle debits and credits csv fields
- Make required options explicit

add nightly

Update register header and readme

Replace pargs with clap to parse args

- includes README updates
- maintains args in separate module

add linux-gnu

Update Pargs version and bump version for release

bump version for release

Merge pull request #13 from ebcrowder/fs

Restructuring