On-disk cache is an unnecessary security risk

[quasistoic]

It creates a cache_dir in your working directory named .op-cache. You will want to delete this after running as it contains all your raw data, including passwords. This was both so I could work offline for initial development and as a workaround for initial data load taking a long time. I will probably get rid of this on-disk cache before distribution.

[quasistoic]

On second thought, the on-disk cache is really useful. Would prefer to instead do smarter cache management:

1. Identify fields that are marked by 1Password as password types and store hashes instead of real values.
2. Prompt the user on exit to indicate whether to clear the on-disk cache or leave it for future use.

[quasistoic]

Added a button to clear the cache in d7b4301