Updated `brew graph`

Changed 'graph' command to show only installed by default. Prettified
resulting graph.

By default, the `brew graph` command would output a dependency graph for every
formula it knew about. Now it only outputs a dependency graph the for formulas
installed. If you want to see the graph for all formulas, use `brew graph
--all`.

Additionally, the old version of the graph command would filter out any
formulas without depdency connections. The updated version now only does this
if calculating dependencies for all formulas via the `--all` flag.

Finally, the resulting graph has been redesigned to be simpler to read. All
formulas which have no other formulas depending on them (i.e., root nodes) are
aligned to the left. They are also outlined in a light grey box, which is
labelled "Safe to Remove". As implied, all of the formulas in this box can be
safely removed without breaking other installed formulas; all formulas outside
this box have at least one installed formula depending on them. This new graph
style is surpressed if the `--all` flag is used.

Closes #18282.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>