Adds a -R (REPL) option to Snabb Switch

Adds a -R option to Snabb Switch which starts an interactive Read-Eval-Print-Loop.

E.g. `sudo ./snabb -R` => interactive joy

The good: It supports the `=<expr>` syntax from LuaJIT's REPL.

The ugly: It can only read single line statements and expressions because I did not wan't to write a Lua tokenizer. The `#luajit` IRC channel on freenode told me there was no portable way to reuse the built-in LuaJIT REPL.