Add ssh_fingerprint (and ssh_fp alias)

Also update README.md

plugins/ssh/README.md

@@ -14,3 +14,10 @@ plugins=(... ssh)
 - `ssh_rmhkey`: remove host key from known hosts based on a host section name from `.ssh/config`.
 - `ssh_load_key`: load SSH key into agent.
 - `ssh_unload_key`: remove SSH key from agent.
+- `ssh_fingerprint` : calculate fingerprint of specifed key files. It has some options:
+  - `-md5` : Use MD5 fingerprint instead of SHA256
+  - `-n` : Don't colorize using ANSI
+  - `-q` : Don't print filename(s)
+  - If key files are not specified, defaults to `~/.ssh/authorized_keys`
+  - `ssh_fp` is an alias to this function
+