Add ability to use a secrets.ini file + jinja templating to apply secrets in cli apps

[lockefox]

• Adds render_secrets() to prosper_config module
  • uses jinja2 templating to obscure secrets
• Adds --secret-cfg switch to CLI template
• Updates get_local_config_filepath() to use os.path utilities rather than just string replace()
• Updates tests for python3.6 (python3.5 might not work any more)

sample credentials.ini file:

[logging]
    discord_webhook = https://discordapp.com/api/webhooks/99999/fake-fake_fake-fake
    slack_webhook = https://hooks.slack.com/services/T1FAKE/B1FAKE/fakefake