src: refactor reading of options in contextify

Refactor various functions that read values from the contextify
options object.  Rather than passing args and the index, pass the
value at that index.

We use env->isolate() rather than args.GetIsolate(), but since env
was constructed from args, this is the same isolate.

PR-URL: https://github.com/nodejs/node/pull/8850


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>