Don't use command flags for submitting password

[RadhiFadlillah]

As version 1.0.1, to create new user we have to run :

fathom register --email="john@email.com" --password="strong-password"

And according to wiki, in latest development version we have to run :

fathom user add --email="john@email.com" --password="strong-password"

This is bad because in Unix it will leave trace in .bash_history. Not to mention because the password is submitted in clear text, people that stand behind our shoulder can look and read it easily.

To fix this, you can use ReadPassword() method in package terminal. This method will accept input from the user (like fmt.Scanln) without echoing it back.

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Since this is simple enough to do, I've created the PR.

[JackEllis]

@RadhiFadlillah Thanks for this, we'll review the PR.