Instaloader

- Support for new target `:saved` to download media marked as saved (78)
- Reimplemented/fixed `Instaloader.test_login()` via graphql query (to fix 76)
- `Post.get_likes()` to iterate over likes of a Post (70)
- Move documentation to https://instaloader.github.io/

- Use UTC in filenames per default (69), maintaining backward compatibility with respect to `--fast-update`
- Fix downloading profile pics (75)
- `--no-profile-pic` to disable profile pic download (61)
- `--no-video-thumbnails` (61)

Reduced GraphQL chunks to 200 per query to fix 59.

- Fix file extension of video stories (54)
- Added `owner_id` and `mediaid` to Post properties (53)
- Allow usage of `{post.<property>}` in filename pattern to access Post properties (53)

- `--metadata-json` flag to export post properties (motivated by 33, 47)
- Post properties (accessible by `--only-if` and exported by `--metadata-json`) now include `caption_mentions` and `tagged_users` (motivated by 47)
- More control about retry mechanism which copes with failing connections: Parameter `--max-connection-attempts`, and possibility to skip retry with a KeyboardInterrupt, i.e. `^C` (motivated by 51)

- Many fixes related to error handling (26, 32, 29, 31)
- `--only-if` option to filter posts (motivated by 15, 24)
- Options `--no-captions` and `--no-geotags` (25)
- Download videos in Sidecars (41)
- Remove command line option `--shorter-output`
- Rename `--skip-videos` to `-no-videos`
- New documentation: https://instaloader.readthedocs.io/
- Major refactorings and code cleanups