Allow _ for unused fields

[Snaipe]

Currently all underscore-prefixed fields raise an error since they might conflict with the internals of wssdl.

However, in lua, it is customary to use _ as a variable name for values we do not care about, e.g. for _, v in ipairs(foo) do ... end. It would be nice to allow _ as a possible field name that would be ignored in the dissection tree -- this would be useful for representing reserved, unused, or padding bytes.