Make update command a soft update opposed to a replacement

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Currently the install command removes the old application and installs the newer version in that location. This means all saved data and configurations will be lost. This is acceptable for the install command.

The word update suggests something less disruptive. The near term (80%) solution is to unzip the downloaded zip to a temporary location and copy the new files overwriting the old files without removing things.

This is not a 100% solution because applications may supply default configurations which would replace a user's preferences saved into the same named config file. A convention would need to exist to balance the needs of an application's user configs against the needs of an application publisher to disruptively change their configuration API. Ultimately, biddle will need to supply a means to identify such breaking changes that should cause warning and user validation during application updates.

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This issue will be resolved by migration to IPFS, see #50