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/common/crypto.rs

@@ -472,6 +472,15 @@ pub fn authenticate(context: &HuskContext, cert: &ValidCert, activate: bool, dom
     Ok(())
 }
 
+pub fn has_certification_capability(cert: &ValidCert) -> bool {
+    cert.keys()
+        .for_certification()
+        .alive()
+        .supported()
+        .revoked(false)
+        .count() > 0
+}
+
 #[cfg(test)]
 pub mod tests {