Add EditorConfig configuration for text editors

Problem/Motivation

Drupal uses 2 spaces for indentation, while Symfony uses 4 spaces for indentation. Switching between the two different formats in your IDE can be bothersome.

Proposed resolution

EditorConfig provides configuration settings for supported text editors on a per-project basis in a hidden .editorconfig file. Opening up a file in Symfony finds Symfony's configuration and uses the 4 space indents, while opening up a file in Drupal will automatically detect the 2 spaced indents.

Remaining tasks

1. Make sure we're using the correct supported properties
2. Should this live in / rather than /core?
3. Get the patch RTBC

User interface changes

None.

API changes

None... This is completely optional. Developers can install the plugin to have their IDE detect the configuration.