Add creating an introducer from a files

- Introducers can now be created by specifying a file. The certificate
  gets imported and declared as an introducer for the passed domains.
- Add some checks, so that the imported certificate can actually be used
  as an introducer (certificate is alive, not revoked, has certification
  capabilities)

src/cli/cli_args.rs

@@ -80,9 +80,18 @@ pub enum IntroducerSubcommand {
 pub struct IntroducerAddCommand {
     #[clap(
         long = "cert",
-        help = "Certificate to declare as introducer."
+        help = "Certificate to declare as introducer.",
+        conflicts_with = "cert_file"
     )]
-    pub cert: String,
+    pub cert: Option<String>,
+
+    #[clap(
+        long = "cert-file",
+        help = "Import certificate from CERT_FILE and declare it as introducer.",
+        value_name = "CERT_FILE",
+        conflicts_with = "cert"
+    )]
+    pub cert_file: Option<String>,
 
     #[clap(
         long = "domains",
@@ -146,7 +155,7 @@ pub struct LocalAddCommand {
         long = "cert",
         help = "Certificate to add."
     )]
-    pub cert: String,
+    pub cert_file: String,
 }
 
 #[derive(Parser, Debug)]