Loop through urls on screen and copy to clipboard

Replace url detection heuristics with a DFA, enabling urls that span
multiple lines. Also fix the selection not to use snapping so that urls
are selected exactly.

st.h

@@ -87,6 +87,7 @@ void printscreen(const Arg *);
 void printsel(const Arg *);
 void sendbreak(const Arg *);
 void toggleprinter(const Arg *);
+void copyurl(const Arg *);
 
 int tattrset(int);
 void tnew(int, int);