Added 'indices' keyword.

- Enables to specify an index (or collection of indices) and only iterate over those indices.
Merge pull request #5 from neocortex/indices-kwarg

While using this, I missed the option to iterate over only specific indices, e.g. if I only want to download 1-grams that start with an 'a'. As it is now, the the iteration goes through all available indices (which is exhausting if, for instance, I don't care about n-grams starting with a digit).

I added a indices keyword to readline_google_store. Default value is None, so not specifying it, everything works as it has, and iteration is over all possible indices.

You can pass it a list of indices like this:

    fname, url, records = next(readline_google_store(ngram_len=1, indices=['z'']))
    fname, url, records = next(readline_google_store(ngram_len=1, indices=['a', 'b']))
    fname, url, records = next(readline_google_store(ngram_len=3, indices=['ab', 'be', 'rt']))

Thanks to @neocortex.