Configuration files

[stephenfin]

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At present, configuration can be loaded from one of three places (in order of priority):

1. Environment flags, e.g. PW_USERNAME
2. Command-line options, e.g. --username
3. The git-config cache, e.g. git config pw.username

There's no reason to break this. However, we should add a fourth mechanism: configuration files. This would allow us to distribute the configuration as part of source control. This configuration file could take the format of pwclientrc files. However, we may also wish to learn from some of the mistakes of that format.

If we do this, we may also wish to add an init sub-command, which would initialize the git-config cache with this data.