Add .editorconfig file

I took a stab at matching current style, but it may not be
comprehensive.

.editorconfig

@@ -0,0 +1,28 @@
+## Reference:
+#   - http://editorconfig.org/
+#   - https://docs.microsoft.com/en-us/visualstudio/ide/create-portable-custom-editor-options
+## Visual Studio IDE support for editing .editorconfig files:
+#   - https://marketplace.visualstudio.com/items?itemName=EditorConfigTeam.EditorConfig
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true
+
+[*.{sln,*proj,*proj.filters,rc,rc2}]
+charset = unset
+end_of_line = crlf
+insert_final_newline = unset
+indent_style = unset
+indent_size = unset
+
+[{configure,configure.*,config.*,*.{sh,yml}}]
+indent_size = 2
+
+[{GNUmakefile,*.{in,opt,vc}}]
+indent_style = tab