Support dev servers defined in a mojoconfig file.

mojoconfig

+# This is a configuration file for devtools (`mojo_run`, `mojo_test) running
+# within a Mojo checkout.
+
+# The content has to parse to a Python dictionary literal. Strings of the form
+# '@{ABC}' are aliases that will be substituted for their values before
+# evaluation:
+#  '@{BUILD_DIR}': path to the output directory
+
+{
+  # Each dev server will be configured as specified and mapped for the
+  # indicated host using --map-origin.
+  'dev_servers': [
+    {
+      'host': 'https://core.mojoapps.io/',
+      'mappings': [
+        ('packages/', '@{BUILD_DIR}/gen/dart-pkg/packages'),

[qsr, 2015/08/04 14:06:45]

As discussed, please use a dictionary for the target paths already as you know
you will want this in a near future.

Also document that the ordering matters.

[ppi, 2015/08/04 16:04:51]

Done (using a list of paths, not a dictionary, as clarified offline).

+        ('', '@{BUILD_DIR}'),
+      ],
+    },
+  ],
+}