Add rustfmt config.

rustfmt.toml

@@ -0,0 +1,14 @@
+imports_granularity = "Item"
+group_imports = "StdExternalCrate"
+
+# In rustfmt 2.0 I will want to adjust these settings.
+#
+# max_width controls the max length of a line before rustfmt gives up
+# but that also scales the length of a bunch of other lines
+# automaticaly due to width_heuristics. I want to find a way to enable
+# rustfmt to work on longer lines when necessary without making my
+# regular code too wide.
+#
+# max_width = 100
+# error_on_line_overflow = true
+# width_heuristics = "Off"