With --metadata-json, a JSON file for each post is created saving the
Post properties defined in instaloader.Post class, i.e. caption, number
of likes, people tagged in caption or the picture itself, etc.
This closes #33 and closes #47.
caption_hashtags is a list of all hashtags that are mentioned in the
Post's caption. It allows to easily filter Posts that have multiple
hashtags, and as such fixes #24.
Further, the documentation of --only-if has been completed by linking to
a description of the syntax in the Python documentation, and by linking
to a list of all defined properties with their meanings. So, this commit
also closes #42.
