restrcit login based on npm client version

[mstorus]

Hi,
I'd like the ability to disallow npm login / npm adduser for certain versions of the npm client.
This information can be read from the version request header.

Use case: I only want to allow login for users who have npm >= 1.5, since npm < 1.5 stores the user's base64-encoded password in plaintext under their local ~/.npmrc file. This is particularly important if you are using the sinopia-ldap plugin, since this password is the user's LDAP password.

Would this be best implemented as a middleware?