Improve graph command to show only installed formulas by default, be prettier

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.