Add an --output argument to sq cert export.

- See #387.

src/cli/cert/export.rs

@@ -1,6 +1,6 @@
 use clap::Parser;
 
-use crate::cli::types::CertDesignators;
+use crate::cli::types::*;
 use crate::cli::types::cert_designator::CertUserIDEmailDomainGrepArgs;
 use crate::cli::types::cert_designator::NoPrefix;
 use crate::cli::types::cert_designator::OptionalValue;
@@ -97,6 +97,14 @@ no search criteria are specified, then this will return success.
     after_help = EXAMPLES,
 )]
 pub struct Command {
+    #[clap(
+        default_value_t = FileOrStdout::default(),
+        help = FileOrStdout::HELP_OPTIONAL,
+        long,
+        value_name = FileOrStdout::VALUE_NAME,
+    )]
+    pub output: FileOrStdout,
+
     #[clap(
         long,
         help = "Emit binary data",

src/commands/cert/export.rs

@@ -10,7 +10,6 @@ use openpgp::{
 use sequoia_cert_store as cert_store;
 use cert_store::Store;
 
-use crate::cli::types::FileOrStdout;
 use crate::{
     Sq,
     print_error_chain,
@@ -28,8 +27,7 @@ pub fn dispatch(sq: Sq, cmd: export::Command) -> Result<()> {
         return Err(anyhow::anyhow!("no query given"));
     }
 
-    let output = FileOrStdout::default();
-    let mut sink = output.create_pgp_safe(
+    let mut sink = cmd.output.create_pgp_safe(
         &sq,
         cmd.binary,
         armor::Kind::PublicKey,