all: implement command "export"

The "export" command export all issuers to file or standard output,

	$ gotp export <FORMAT> [FILE]

List of known supported FORMAT is: uri.
If FILE is not defined it will print to standard output.
The list of exported issuers are printed in order of its label.

1 files changed, 7 insertions, 0 deletions

diff --git a/README.md b/README.md
index 13e972f..13f6e80 100644
--- a/README.md
+++ b/README.md
@@ -26,6 +26,13 @@ seconds, default to 30 seconds.
 The ISSUER field is also optional, its define the name of
 provider that generate the secret.
 
+    export <FORMAT> [FILE]
+
+Export all the issuers to file format that can be imported by provider.
+Currently, the only supported FORMAT is "uri".
+If FILE is not provided, it will print to the standard output.
+The list of exported issuers are printed in order by its label.
+
     gen <LABEL> [N]
 
 Generate N number passwords using the secret identified by LABEL.