config: add grapheme-width-method=default

This is the new default mode, and tries to do "the right thing". That
is, recognize and handle:

* Variation Selector 16 (graphical presentation)
* ZWJ sequences
* Skin tones and hair-styles
* Country flags

Things known to be incorrect:

* "Invalid" country letter combinations are still treated as
  flags. That is, the combined width is 2, even though it should be 4.

config.h

@@ -326,6 +326,7 @@ struct config {
         bool overflowing_glyphs;
         bool grapheme_shaping;
         enum {
+            GRAPHEME_WIDTH_DEFAULT,
             GRAPHEME_WIDTH_WCSWIDTH,
             GRAPHEME_WIDTH_DOUBLE,
             GRAPHEME_WIDTH_MAX,