Construct a CountedVocabulary out of a vocabulary file.

    Note:
      File has the following format word1 count1
                                    word2 count2

def from_vocabfile(filename):
    """ Construct a CountedVocabulary out of a vocabulary file.

    Note:
      File has the following format word1 count1
                                    word2 count2
    """
    word_count = [x.strip().split() for x in _open(filename, 'r').read().splitlines()]
    word_count = {w:int(c) for w,c in word_count}
    return CountedVocabulary(word_count=word_count)