Creates a new flutter command 'ide-config' and removes *.iml and .idea from tree. (#12958)

Creates a new (hidden) flutter command 'ide-config' that will create and/or update
existing .iml files and some files under the .idea directory, as well as
removing existing *.iml files and the .idea directory.

It also:

 * Adds *.iml to the .gitignore
 * Removes existing .iml files from the repo, and moves them to the
   packages/flutter_tools/ide_templates/intellij directory.
 * Adds a flag to ide-config ('--update-templates') that will take any new .iml
   files in the flutter tree and add them to the existing templates.
     - If --overwrite is also specified, then all existing templates will also
       be overwritten with the contents from the flutter tree, and any that have
       been deleted from the flutter tree will also be removed from the
       templates.
 * Added new run configurations for all existing app targets that will now also
   be automatically added to IntelliJ.
 * Setting up the environment also includes setting the coding style guidelines
   and the git VCS.
 * Note that after this PR lands, Flutter developers will need to run it once to
   re-create the .iml files and configuration files that have been removed.

After this PR lands, .iml files will no longer appear in the untracked files
section for git.

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 9d0adf1..db2baf6 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -44,14 +44,17 @@
  * Run `flutter update-packages` This will fetch all the Dart packages that
    Flutter depends on. You can replicate what this script does by running
    `pub get` in each directory that contains a `pubspec.yaml` file.
+ * If you plan on using IntelliJ as your IDE, then also run
+   `flutter ide-config --overwrite` to create all of the IntelliJ configuration
+   files so you can open the main flutter directory as a project and run examples
+   from within the IDE.
 
 Running the examples
 --------------------
 
-To run an example with a prebuilt binary from the cloud, switch to that
-example's directory, run `pub get` to make sure its dependencies have been
-downloaded, and use `flutter run`. Make sure you have a device connected over
-USB and debugging enabled on that device.
+To run an example, switch to that example's directory, and use `flutter run`.
+Make sure you have an emulator running, or a device connected over USB and
+debugging enabled on that device.
 
  * `cd examples/hello_world`
  * `flutter run`