Introduce option  --guess-language-eagerly

This is a fix for issues discussed in " regression: getFileLanguage logic is reversed #123".
Here you're running `extractInterpreter()` a second time.  You can optimize by moving this taster to the first in the `eager_tasters` array and passing a smaller array if `extractInterpreter()` has just been run, something like this maybe:

~~~c
language = tasteLanguage(&glc, &eager_tasters[!!status->isExecutable],
                            ARRAY_SIZE(eager_tasters) - !!status->isExecutable)
~~~