src: add type check to v8.setFlagsFromString()

Calling v8.setFlagsFromString with e.g a function as a flag argument
gave no exception or warning that the function call will fail.

There is now an exception if the function gets called with the wrong
flag type (string is required) or that a flag is expected.

Other APIs already provide exceptions if the argument has not the
expected type.

PR-URL: https://github.com/iojs/io.js/pull/1652


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>