Allow passing arbitrary command line flags to klee and kleaver when running llvm-lit

This is done by doing something like

```
$ cd test/
$ llvm-lit --param klee_opts=--xxx --param kleaver_opts=--yyy .
```

This would pass ``--xxx`` as an extra option to ``klee`` when running
tests and would pass ``--yyy`` as an extra option to ``kleaver`` when
running tests.

This feature has been added to make it easy to pass extra flags to
KLEE or Kleaver without needing to modify tests or recompile KLEE with
different defaults (yuck!).