Add image alt text, fixes #98

[JasirZaeem]

This PR fixes issue #98 I needed to make these changes to fix issue #91 so I will wait for this to be merged before I proceed with that.

Changes:

• Changed tweet_labels to a dict, random label is chosen from the dict keys, will add definitions in the next PR.
• Added get_tweet_content, return the tweets text and its label as a dict.
• Added get_img_for_tweet, takes the texts of the tweet, generates a random image, returns location of the generated image
• Updated tweet.py to post the tweet along with an image which has its alt text, and then delete the image once posted.

[JasirZaeem]

I have also created a directory "img_post" where the generated images a stored, and it has a gitignore file so that those files aren't indexed by git as they are temporary

[VickiLanger]

Thanks for your contribution.

---

Don't forget to follow the bot on Twitter to see the hard work you put in.