Add ability to encrypt CA private key at rest (#386)

Fixes #8.

`nebula-cert ca` now supports encrypting the CA's private key with a
passphrase. Pass `-encrypt` in order to be prompted for a passphrase.
Encryption is performed using AES-256-GCM and Argon2id for KDF. KDF
parameters default to RFC recommendations, but can be overridden via CLI
flags `-argon-memory`, `-argon-parallelism`, and `-argon-iterations`.

cmd/nebula-cert/passwords.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"golang.org/x/term"
+)
+
+var ErrNoTerminal = errors.New("cannot read password from nonexistent terminal")
+
+type PasswordReader interface {
+	ReadPassword() ([]byte, error)
+}
+
+type StdinPasswordReader struct{}
+
+func (pr StdinPasswordReader) ReadPassword() ([]byte, error) {
+	if !term.IsTerminal(int(os.Stdin.Fd())) {
+		return nil, ErrNoTerminal
+	}
+
+	password, err := term.ReadPassword(int(os.Stdin.Fd()))
+	fmt.Println()
+
+	return password, err
+}