Feature: 'explode' a directory

In the following scenario:
```
- foo (2G)
|- bar (1.5G)
\- baz (0.5G)
- qux (1.7G)
- quux (0.4G)
```
diskonaut would show 3 blocks (foo, qux and quux). After entering `foo`, it would show 2 blocks (bar and baz).

It would be neat to have an option to 'explode' foo in this case, resulting in a view that shows the relative sizes of bar, baz, qux and quux.

No files, tests, or entry points are named in the issue. Start by locating the directory navigation and terminal visualization entry points, then determine how an option could combine foo's children with qux and quux; done means a view showing bar, baz, qux, and quux with their relative sizes.

Written by the indexing model from the issue text.