Add credential import/export commands

[JustinAzoff]

addcredential works for one at a time, but should support importing a list from a CSV or TSV file.

[JustinAzoff]

Done, implemented TSV and JSON importing under the cred subcommand

addcredential is now cred add, along with

Available Commands:
  add         add a new credential pair
  list        list credentials
  import      load credentials from TSV or JSON

$ cat /tmp/creds.tsv 
root	root	14
test	test	30
$ cat /tmp/creds.json 
{"User":"root","Password":"root","ScanInterval":14}
{"User":"test","Password":"test","ScanInterval":14}
{"User":"ubnt","Password":"ubnt","ScanInterval":14}
{"User":"guest","Password":"guest","ScanInterval":2}
$ ./ssh-auditor cred import tsv < /tmp/creds.tsv 
INFO[02-25|15:37:51] added credential                         user=root password=root interval=14
INFO[02-25|15:37:51] added credential                         user=test password=test interval=30
$ ./ssh-auditor cred import json < /tmp/creds.json 
INFO[02-25|15:38:01] updated credential                       user=root password=root interval=14
INFO[02-25|15:38:01] updated credential                       user=test password=test interval=14
INFO[02-25|15:38:01] added credential                         user=ubnt password=ubnt interval=14
INFO[02-25|15:38:01] added credential                         user=guest password=guest interval=2