Make lpeg an optional dependency

[jacobdufault]

If require "lpeg" fails then LuLPeg is used instead.

[andremm]

Have you tested with Lua 5.3?

[jacobdufault]

Looks like there are a few issues - load/loadstring is easily fixed, but when running tests I receive this error:

$ ../lua53.exe test.lua
> testing lexer...
expected: { `Local{ { `Id "h" }, { `Number "1900" } }, `Local{ { `Id "i" }, { `Number "4294967296" } } }

got: { `Local{ { `Id "h" }, { `Number "1900" } }, `Local{ { `Id "i" }, { `Number "0" } } }

C:\Users\jacob_000\Desktop\Lua\lua.exe: test.lua:181: assertion failed!
stack traceback:
        [C]: in function 'assert'
        test.lua:181: in main chunk
        [C]: in ?