Return and seach for word categories

[claw89]

Supersedes #80

For my use case, I need to obtain information on all words in a particular category. For example, I might need to collect etymology information for all English words derived from the Bible.

The two main additions of this PR are:

• An optional argument "return_categories" in parser.fetch(): when return_categories=True, parser.fetch() returns the word information and a list of the associated categories.
• A new function parser.fetch_category(): this function returns a list of the words organized under the specified category.

Using these addition, the above use case can be completed with the following code:

bible_words = parser.fetch_category('English terms derived from the Bible')
for word in bible_words:
    print("==============================")
    print(word)
    word_info = parser.fetch(word)
    for item in word_info:
        print(item['etymology'])