feature: help usage only display usage information for command-line flags

[FranciscoKurpiel]

Description

When proteus generates usage information, like when the program is called with --help, it currently shows only usage with command-line flags. There is no information about how to provide parameters using other providers, like environment variables or consul KV store.

Goals

From the usage information generated with --help:

• the user will know what providers can be used to configure the application, and the priority order
• the user will know how to provide each parameter with each provider
• (tentative): the provider can give a short documentation about itself

Steps:

• RFC
• Implementation