DHE: Export keypair to DH format file

[bmingles]

The extension will manage public/private key pairs after #32 is merged. It might be useful to be able to export keypairs into a DH format key file that can be used for other use cases outside of the extension. DHE: Import DH keypair for use by the extension #160 will handle the import counterpart.

[mofojed]

I don't think as useful, as you can always generate the keypair outside of VSCode and upload that public key; the key generated/used within VSCode doesn't really need to be used elsewhere.