Add mix task to display artifact name/information
Add mix task to display artifact name/information

Fixes #771

This fix adds a mix task `mix nerves.artifact.name` which display various artifact  names/information w/o triggering a artifact re-build.

Example:
```
$ mix nerves.artifact.name
==> nerves
==> nerves_system_bbb
Generated nerves_system_bbb app

Nerves environment
  MIX_TARGET:   target
  MIX_ENV:      dev

Name:               nerves_system_bbb-portable-2.15.1
Download Name:      nerves_system_bbb-portable-2.15.1-0C8DD7C
Download File Name: nerves_system_bbb-portable-2.15.1-0C8DD7C.tar.gz
Download Path:      /home/udos/.nerves/dl/nerves_system_bbb-portable-2.15.1-0C8DD7C.tar.gz
Checksum:           0C8DD7CEB69659D7486BBA6B22AE1CB498EF3487A5369411F0C157477F8FF2D0
```

nerves-project/nerves,nerves-project/nerves