Added -S argument for better stack traces.

Uses StackTracePlus: https://github.com/ignacio/StackTracePlus

New look with -S:

    $ sudo ./snabb -S -e 'assert(nil)'
    [string "assert(nil)"]:1: assertion failed!
    line not found

    Stack Traceback
    ===============
    (1) Lua function 'handler' at file 'core/main.lua:137' (best guess)
	    Local variables:
	     reason = string: "[string \"assert(nil)\"]:1: assertion failed!"
	     (*temporary) = C function: print
    (2) global C function 'assert'
    (3) main chunk of [string "assert(nil)"] at line 1
    (4) Lua function 'main' at file 'core/main.lua:66' (best guess)
	    Local variables:
	     args = table: 0x53587ff0  {1:-S, 2:-e, 3:assert(nil)}
	     i = number: 2
	     thunk = Lua function '?' (defined at line 0 of chunk "assert(nil)"])
	     error = nil
    (5) global C function 'xpcall'
    (6) main chunk of file 'core/main.lua' at line 142
    (7) global C function 'require'
    (8) main chunk of [string "require "core.main""] at line 1
	    nil

Old look:

    $ sudo ./snabb -e 'assert(nil)'
    [string "assert(nil)"]:1: assertion failed!
    stack traceback:
	    core/main.lua:137: in function <core/main.lua:135>
	    [C]: in function 'assert'
	    [string "assert(nil)"]:1: in function 'thunk'
	    core/main.lua:66: in function <core/main.lua:44>
	    [C]: in function 'xpcall'
	    core/main.lua:142: in main chunk
	    [C]: in function 'require'
	    [string "require "core.main""]:1: in main chunk