Add an editorconfig file (libusual#60)

This adds an `.editorconfig` file to help maintain a consistent coding
style. It's copy-pasted from the PgBouncer repo, with some PgBouncer
specific files removed.

Related to libusual#59

.editorconfig

@@ -0,0 +1,21 @@
+# top-most EditorConfig file
+root = true
+
+# basic rules for all files
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+trim_trailing_whitespace = true
+
+[*.{c,h}]
+indent_style = tab
+indent_size = 8
+
+[*.py]
+indent_style = space
+indent_size = 4
+
+[*.{yml,sh}]
+indent_style = space
+indent_size = 2