fix: address bugs with "uknown-options-as-args"

[mleguen]

Closes #226.

Fixes 3 bugs in hasAnyFlag:

1. using ... inside concat flattens all arrays. So if flags.string (for example) is an array of string, toCheck does not contain this array of string, but the strings directly. And a few lines below, if flag is a string, flag[repeat] evaluates to true (as with any method names of String.prototype)
2. once corrected the first bug, if flag is now an array, flag[some] evaluates to true (Array.prototype...)
3. same thing for the methodes of Object.prototype if flag is an object

By the way, yargs-parser crashes when passing a --toString option: I thing this kind of bug has to be tracked everywhere in the code (I will open another issue for this).