New Plugin: yubikey

The `yubikey` plugin provides some very powerful features:

  * Shared ssh-agent among zsh sessions
  * Easy-to-use aliases
  * Autoconfiguration where possible

plugins/yubikey/README.md

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+# YubiKey Plugin
+
+YubiKey plugin -- Provides aliases to help use YubiKey tokens comfortably
+
+## Usage
+
+This plugin will first try to detect location of the 'opensc-pkcs11.so' library, unless already specified in the $OPENSC env var (that is, before source-ing oh-my-zsh).
+
+Afterwards, it will try to detect if a 'shared ssh-agent' is already running, through a file in `/run/user/$UID`, or if no such directory, in `/tmp` as a fallback.
+
+Then it will define several aliases. You can see them by issuing the command `alias | grep yubi-`.
+
+## Optional Parameters
+
+These parameters can be set **_before_** source-ing oh-my-zsh to customize the settings:
+
+`YUBI_SHOWKEYS`
+> If set to '1' or 'y' or 'yes' (case-insensitive), will list the keys contained in the 'shared ssh-agent'
+
+`YUBI_SSHAGENT_AUTOINIT`
+> If set to '1' or 'y' or 'yes' (case-insensitive), will automatically initialize the 'shared ssh-agent' if one is not found
+