Improve the CLI output and interactions

[stayallive]

New CLI output:

./sentry-agent help
Sentry Agent 1.2.3 

Description:
  A local agent that listens for Sentry SDK requests and forwards them to the destined Sentry server.
  
Usage:
  ./sentry-agent [options]
  
Options:
  -h, --help                            Display this help output
      --listen=ADDRESS                  The address the agent listens for connections on [default: "127.0.0.1:5148"]
      --upstream-timeout=SECONDS        The timeout for the connection to Sentry (in seconds) [default: "2.0"]
      --upstream-concurrency=REQUESTS   Configures the amount of concurrent requests the agent is allowed to make towards Sentry [default: "10"]
      --queue-limit=ENVELOPES           How many envelopes we want to keep in memory before we start dropping them [default: "1000"]
  -v, --verbose                         When supplied the agent will print debug messages to the console, otherwise only errors and info messages are printed

Use "proper" flags for giving options (this is a breaking change).

Added a --verbose flag to keep the output a little cleaner and not show success messages since they are basically useless unless you want to see what it's doing.

We also show the version number for tagged versions, shows 0.0.0 when developing or running local builds. Added it to the help output and the startup string so we can hopefully identify the version of the agent the user is actually running.

This is all implemented extremely basic on purpose.

[Litarnus]

Nice!