Tools: Update editorconfig to make github diffs easier to read

Github uses .editorconfig, if present, to determine how wide the tab character should be in diffs. See isaacs/github#170. Right now, it's using the default of 8, which makes diffs a pain to read.

This changes the value to 2, which should make diffs easier to read by letting more of the diff appear on screen. Arguments could be make (and will be I bet) about the correct value (4! 2! 3!) but getting off 8 would be nice.

.editorconfig

@@ -2,7 +2,13 @@
 root = true
 
 [*]
+# yes, we use tabs
 indent_style = tab
+
+# To control tab size in github diffs
+# See https://github.com/isaacs/github/issues/170#issuecomment-150489692
+indent_size = 2
+
 end_of_line = lf
 charset = utf-8
 trim_trailing_whitespace = true