Allow name_only option gensim downloader api

[aneesh-joshi]

Currently, to get the exact names of the models or corpora, a user has to either:

1. run gensim.info() and look through the huge json dump to get the exact names
2. go to the gensim-data website and check

When using gensim-data, I often forget the exact key.
"Was it 'glove-wiki-gigaword' or 'glove-gigaword-wiki'?"

It would be very helpful if a user could, in the terminal or otherwise, type:
gensim.info(name_only=True) or
python -m gensim.downloader --info_name_only
ans get the following output:

{
    "corpora": [
        "semeval-2016-2017-task3-subtaskBC",
        "semeval-2016-2017-task3-subtaskA-unannotated",
        "patent-2017",
        "quora-duplicate-questions",
        "wiki-english-20171001",
        "text8",
        "fake-news",
        "20-newsgroups",
        "__testing_matrix-synopsis",
        "__testing_multipart-matrix-synopsis"
    ],
    "models": [
        "fasttext-wiki-news-subwords-300",
        "conceptnet-numberbatch-17-06-300",
        "word2vec-ruscorpora-300",
        "word2vec-google-news-300",
        "glove-wiki-gigaword-50",
        "glove-wiki-gigaword-100",
        "glove-wiki-gigaword-200",
        "glove-wiki-gigaword-300",
        "glove-twitter-25",
        "glove-twitter-50",
        "glove-twitter-100",
        "glove-twitter-200",
        "__testing_word2vec-matrix-synopsis"
    ]
}

Notes:
The current develop's downloader.py is failing the doctests without me doing anything.

[menshikh-iv]

better to do it as parameter of --info flag I think (instead of new --info_* flag), like --info name

[aneesh-joshi]

changes made @menshikh-iv

[menshikh-iv]

--info name please :) (but stay name_only parameter for CLI)

[menshikh-iv]

@aneesh-joshi thanks!