Make Whippet respect semver constraints

[RobjS]

Currently, Whippet can either:

a) Just install the latest available version of a given plugin; or
b) Install a single specific version (based on git tag) of a specific plugin.

It would be very useful to be able to use semver constraints in whippet.json, so that we could e.g. specify a particular major version, allowing minor version updates when whippet deps update is run, with an alert if a new major version is available. The new major version would only be installed if the constraints in whippet.json were updated to allow that.