Support setting custom URI scheme flags

[aperezdc]

The WebKit API allows setting certain properties for custom URI schemes, using the following functions:

• webkit_security_manager_register_uri_scheme_as_local()
• webkit_security_manager_register_uri_scheme_as_no_access()
• webkit_security_manager_register_uri_scheme_as_display_isolated()
• webkit_security_manager_register_uri_scheme_as_secure()
• webkit_security_manager_register_uri_scheme_as_cors_enabled()
• webkit_security_manager_register_uri_scheme_as_empty_document()

It would be nice of we would support setting those as part of cog_shell_set_request_handler(), and also allowing setting those properties as flags passed to the --dir-handler= option of the launcher, for example like this:

cog --dir-handler=myscheme/local,secure:/path/to/files ...