Improved enterprise configuration

Breaking changes:

• The default session_type no longer uses LastPass for GCP/Google Sheets credentials. Set session_type to "lpass" for that mode - see the new configuration file setting in CONFIG.md to make that configuration permanent.
• Stricter hint validation is done internally, which may affect use of undocumented date/time formats--please file an issue if this affects a date/time hint value you rely on. (#66)

New features:

• Allow session type to be configured via config file, enabling a long-term default for which secret management system to use for credentials (#70)
• Allow IAM username suffix to be added to S3 scratch URL, enabling enterprise configurations using a common scratch bucket (#69)
• All hints can now be configured via CLI arguments on both input and output--notably, this allows a header row to be configured on output! (#66)

Bug fixes / reliability improvements:

• Minor error message tweaks (#71)