Add string group to syntax files

This commit adds the string group (constant.string) to valid strings
in all the syntax files that support colorschemes (see the readme in
the runtime/syntax directory).

Fixes #178

runtime/syntax/vi.micro

@@ -4,5 +4,5 @@ color identifier "[A-Za-z_][A-Za-z0-9_]*[[:space:]]*[()]"
 color statement  "\b([nvxsoilc]?(nore|un)?map|[nvlx]n|[ico]?no|[cilovx][um]|s?unm)\b"
 color statement  "\b(snor|nun|nm|set|if|endif|let|unlet)\b"
 color statement "[!&=]"
-color constant ""(\\.|[^"])*"|'(\\.|[^'])*'"
+color constant.string ""(\\.|[^"])*"|'(\\.|[^'])*'"
 color comment  "(^|[[:space:]])\"[^"]*$"