revid-cli: make sender destinations use URI syntax

[kortschak]

At the moment we hand revid-cli two parameters for each destination. With the addition of more than one destination, it would make sense to bundle these pairs of destination type and target into a single parameter. An obvious way to achieve this would be to use a URI, taking the scheme as the target type (i.e. using it as it was intended to be used).

We can still use a path; a destination without a scheme is parsed as a path, and the absent scheme can be interpreted as a file://.