Add support for specifying scheme and ssl config for proxy target

This commit adds some additional fields that you can specify
when configuring a proxy target; specifically, you may now
include a `:scheme` to force http or https (rather than using
the scheme of the original request).  You can also specify
an `:ssl-config` map, to specify the pem files to use for
the ssl context.
Add docs for new `add-proxy-route` function