More control over debug information

* added one more cmdline option -debug-level which
  controls the verbose level of debug message. The larger
  the number is, the more verbose message is generated.
  The default level is 1.

* when the debug-level is larger than 1, then dump all
  replacements that collected by souper before they are processed
  by KLEE. The existing debug-souper behavior will not be affected
  if we do not pass a level higher than 1.