Increase verbosity on Heroku (-vv)

[wilzbach]

This will log debug messages on Heroku as well.

• We have a free logging addon that allows to search logs anyway
• In case of a misbehavior more information is always appreciated
• Usually only the last few hours, maybe days are relevant anyhow

Vibed:

 -v --verbose        Enables diagnostic messages (verbosity level 1).
    --vv, --vverbose Enables debugging output (verbosity level 2).
    --vvv            Enables high frequency debugging output (verbosity level
                     3).
    --vvvv           Enables high frequency trace output (verbosity level 4).

Verbosity level 2 is LogLevel.debug_:

https://github.com/rejectedsoftware/vibe.d/blob/master/core/vibe/core/log.d#L173

[dlang-bot]

Thanks for your pull request, @wilzbach!

[PetarKirov]

Ok, let's give it a shot.