Move ASCII graph to `--graph` switch, and add a `--tree` alternative & fallback ...

Move ASCII graph to `--graph` switch, and add a `--tree` alternative & fallback (#465)

* New `alr with --tree`, `alr show --tree` switch

Prints dependency graph in tree form, as an alternative to the ascii-art plots
that 1) not always will be available, depending in libgraph-easy-perl and 2)
quickly become big and broken.

* Move ASCII graph to dedicated --graph switch

Previously, it was always attempted to be shown with `--solve`, which could be
an unnecessary pollution for an otherwise useful command.

* Update user-changes document