allow specification of multiple certificate fingerprints on command-line via --servercert

allow specification of multiple certificate fingerprints on command-line via --servercert

Server certificates will be accepted if they match *any* of the provided fingerprints.

Behavior with `--servercert` is otherwise unchanged; it still disables system trust
stores, meaning that _only_ certificates matching the provided fingerprints will be
accepted if it is specified one or more times.

This will allow the use of `--servercert` to non-interactively connect to a server which
has a non-trusted certificate and redirects to one or more other servers with non-trusted
certificates. (See #25 for a real case.)

Signed-off-by: Daniel Lenski <dlenski@gmail.com>