Add support for copy/paste registers

This adds support for vim-style copy/paste registers which allows
cutting and copying multiple items without having to overwrite the
copy/paste buffer. Registers can be specified using the quote key ('"').
To access a register, type '"x' before a command where "x" is the name
of a register. If you want to copy the currently selected item into
register 1, type '"1c'.

Valid registers are 0-9, a-z, "-" and "_". Note that the latter is the
so-called black hole register, which works similar to the black hole
register in vim.

The register prefix key is currently hardcoded and cannot be configured.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/help.c

@@ -800,7 +800,7 @@ void help_screen(void)
     }
 
     wins_scrollwin_display(&hwin);
-    ch = keys_getch(win[STA].p, NULL);
+    ch = keys_getch(win[STA].p, NULL, NULL);
   }
   wins_scrollwin_delete(&hwin);
   if (need_resize)