cli: add pipeline show --tree

[puhoshville]

Add option to unfold dependencies into a tree.

Fixes #1038

[efiop]

Great patch! 🎉 A few comments down below. Also, would you mind showing us an example of dvc pipeline show --tree output?

[puhoshville]

Great patch! 🎉 A few comments down below. Also, would you mind showing us an example of dvc pipeline show --tree output?

Example of dvc pipeline show --tree output
For following graph
dvc pipeline show --ascii e.file.dvc

+------------+
| a.file.dvc |
+------------+
       *
       *
       *
+------------+
| b.file.dvc |
+------------+
       *
       *
       *
+------------+              +------------+
| c.file.dvc |              | d.file.dvc |
+------------+              +------------+
             ***           ***
                **       **
                  **   **
              +------------+
              | e.file.dvc |
              +------------+

we got following tree:
dvc pipeline show --tree e.file.dvc

e.file.dvc
├── c.file.dvc
│   └── b.file.dvc
│       └── a.file.dvc
└── d.file.dvc

[efiop]

@puhoshville Thanks! :) Could you also address this one #1781 (comment) please? You just need to raise DvcException instead of ValueError.

[efiop]

@puhoshville Thank you so much! 🎉