Add method to get title of post (#969)

[17it151tejaschauhan]

For the solution of issue #969

-- A motivation for this change

• Fixes #
• Some post of instagram has title as same as caption but current version of instaloader does not give title of post.
• To get title of post, I've added one property method called title as same as caption method in class Post.

-- The completeness of this change

• In json response of graphql query, it gives title of post with key name as title as same as location in value of shortcode_media
  but instaloader was not considering it.

• Added one method named title in class Post which will get value of title from _full_metadata_dict dictionary.

• If title is not present in json response, it will return None.

• Need to update documentation for this method.

• I consider it for ready to be merged.

• Please guide me if necessary.

• Example of use case:

from instaloader.instaloader import Instaloader, Post
L = Instaloader()
post = Post.from_shortcode(L.context, "CKA_09eMMVE")
print("Title of Post : ", post.title)

[aandergr]

Thanks! Merged for upcoming version 4.7.

[17it151tejaschauhan]

Thanks! Merged for upcoming version 4.7.

Thanks for such a great library.