Don't print out delete character for password field

[jjcarstens]

This is more of a nitpick/quirky behavior. But say you're like me and always fat finger passwords then have to delete back a few characters or ctl-u the whole line. Those characters currently get printed out and create new lines for the password prompt:

jonjon@ 📂 {nerves_hub} $ mix nerves_hub.firmware list --product smartrent_hub_fw
NervesHub server: 0.0.0.0:4002
NervesHub organization: jonjon
Local NervesHub user password: ^R
Local NervesHub user password: ^R
Local NervesHub user password: ^R
Local NervesHub user password: ^U
Local NervesHub user password: 

It still works and deletes those characters. Just odd to see if all get printed out multiple times. Would be nice to have it work silently like other password prompts at the terminal