UX improvements

[cdstanford]

From discussion today,

• [x] Make audit the default binary, rename scan to scan_csv
• [x] For audit: add a default argument for <AUDIT_FILE_PATH>
• [x] Provide a bit more info to stdout: continuing an existing audit, saving audit to ,etc. (tool already does a good job of some of this, telling you it's loading an audit file, creating a new audit file)
• [x] Provide a way to run audit and create a new scan, listing all effects (without auditing -- matching the behavior of scan)

Currently the -r flag is supposed to do the third thing, but I can't quite get it to do what I want. Maybe the solution is just, if the audit file doesn't exist, create it with the default scan?

caleb@caleb-mac cargo-scan % cargo run --bin audit data/test-packages/fns-closures audit.audit2 -r
    Finished dev [unoptimized + debuginfo] target(s) in 0.40s
     Running `target/debug/audit data/test-packages/fns-closures audit.audit2 -r`
Error: Audit file to review doesn't exist
caleb@caleb-mac cargo-scan % 
caleb@caleb-mac cargo-scan % 
caleb@caleb-mac cargo-scan % cargo run --bin audit data/test-packages/fns-closures audit.audit3 -r
    Finished dev [unoptimized + debuginfo] target(s) in 0.53s
     Running `target/debug/audit data/test-packages/fns-closures audit.audit3 -r`
Error: Audit file to review doesn't exist

[DavidThien]

Finished with this pending a decision on if we need to have a better output format for the preview behavior (list all effects and don't save to an audit file).

[cdstanford]

woohoo