Add support for instagram.com user profiles and pages (#134)

* [instagram] Add extractor for instagram.com user profiles and pages

The extractor scrapes `instagram.com/<user>' timelines and
`instagram.com/p/<shortcode>' by mimicking the behaviour of a web
browser and extracting the sharedData JSON of the single pages.

Please note that this mean that for user timelines we also do an
extra request to the `instagram.com/p/<shortcode>' page but this
permit to have consistent (and all) information about the media
fetched.

The MD5 logic used for X-Instagram-GIS was documented in

 <https://stackoverflow.com/questions/49786980/>

* [instagram] Test for keywords, not url for GraphImage and GraphSidecar

URLs returned by instagram seems not stable so avoid testing for
them and instead test for keyword returned.

* [instagram] Improve test of InstagramProfilepageExtractor

Also check the count of media returned.

* [instagram] Several cleanup and improvements

- Change description, subcategories to generate a better description in
  docs/supportedsite.rst
- Remove not needed InstagramExtractor.__init__()
- Use text.parse_int() instead of directly using int() (the former is more
  robust)
- Use self.request().json() instead of using json.loads() the
  self.request().text()
- Add `pattern:' to check the URLs where we do not have a stable URLs.
  It seems that only the subdomain is not stable.

Thanks to @mikf!

gallery_dl/extractor/__init__.py

@@ -45,6 +45,7 @@ modules = [
     "imgbox",
     "imgth",
     "imgur",
+    "instagram",
     "jaiminisbox",
     "joyreactor",
     "khinsider",