[gfycat] support when accidentally prodiving media uri

[AnhNhan]

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• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

When pasting gfycat URLs copied from the browser, or pasting URLs shared via chat/social media, links are often provided as direct media urls, e.g.

https://thumbs.gfycat.com/acceptablehappygoluckyharborporpoise-size_restricted.gif
https://giant.gfycat.com/acceptablehappygoluckyharborporpoise.mp4

youtube-dl had downloaded these URLs using the generic extractor, though using the gfycat extractor would yield the best results (and not simply download the reduced gif preview).

I have verified that my changes work by adding tests and running tests/test_all_urls.py and also running it myself on the Terminal.

C:\Users\anhnh\dev\youtube-dl (gfycat-contenturls)
λ python -m youtube_dl https://thumbs.gfycat.com/DopeyBigAfricanclawedfrog-max-1mb.gif
[Gfycat] DopeyBigAfricanclawedfrog: Downloading video info
[download] Destination: a saerom-fromis 180727 [UeBMgvOQmR8]-9-DopeyBigAfricanclawedfrog.mp4
[download] 100% of 663.43KiB in 00:01

C:\Users\anhnh\dev\youtube-dl (gfycat-contenturls)
λ python -m youtube_dl https://giant.gfycat.com/DefinitePlayfulAmazonparrot.webm
[Gfycat] DefinitePlayfulAmazonparrot: Downloading video info
[download] Destination: saerom-fromis 180727 [UeBMgvOQmR8]-6-DefinitePlayfulAmazonparrot.mp4
[download] 100% of 203.81KiB in 00:01

Note that the gfycat extractor has been selected and the info dict provided the video title.