Add the ability to store personalized commands

[gcampax]

User story: As a user, I want my agent to remember all commands that I was able to use successfully, so that in the future I will be able to reissue the same command identically without fear that the command would be misinterpreted due to a model update.

Implementation: We can probably leverage the exact matcher we use for developer mode, together with a plain text database + binary trie index stored in the user's directory.