Add .editorconfig file

A .editorconfig file allows many editors (e.g. Visual Studio Code)
to autoconfigure themselves based on its contents. This avoids
some formatting issues in PRs because editors will do the right
thing by default with a .editorconfig file available in the repo.

.editorconfig

@@ -0,0 +1,10 @@
+root = true
+
+[*]
+charset = utf-8
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[{CMakeLists.txt,*.cmake,*.rst}]
+indent_size = 2
+indent_style = space

.gitattributes

@@ -1,5 +1,6 @@
 .git*            export-ignore
 .hooks*          export-ignore
+.editorconfig    export-ignore
 
 # Custom attribute to mark sources as using our C code style.
 [attr]our-c-style  whitespace=tab-in-indent  format.clang-format-6.0