Refactor and improve web-search plugin

[mcornella]

STATUS: ready!

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This pull request implements simpler logic that enables adding more search engines in the future, while mantaining ease-of-use to define aliases.

It also implements the following very solicited features:

• Silence nohup:
  Fix [plugin][web-search] fix + changes #1970, fix web-search plugin: unwanted output to nohup.out #3097, fix Update web-search plugin #3183, fix update web-search to not write to nohup.out #3218, fix [web-search] Redirect output of nohup command to /dev/null #3302, fix stderr redirection in web-search plugin #3441.
• Add Yandex support:
  Fix Plugin web-search: added Yandex search engine #1845, fix add yandex support to web-search plugin #3119.
• Add Cygwin support to open command:
  Fix add support of CYGWIN in the web-search #2831, fix Add Cygwin support to web_search #3103.

In total, 10 issues can be closed with this PR.
Feedback is greatly appreciated.

[mcornella]

@ajdle @Xenopathic @alessandro-pezzato @bchretien @takac @mkolosick @varunkvv @ibnHatab @biriukov @ai81 @mmounirou @friederbluemle @peteches @Guerki @stibinator @stelim @xuhdev
Can you guys test this out?

[varunkvv]

👍 . Tested on ubuntu with zsh 5.0.2.

Edit: Plus this makes it so easy to search for documentation with ddg aliases or github or stackoverflow. :)

[xuhdev]

No problem on 5.0.7 on Arch Linux. There is one thing noticeable: the yandex search is using http but others are using https. I tried yandex with https which also works. Don't know if there is a specific reason to use http in this case.

[mcornella]

Huh, that's weird. I never checked the search page, but the home page redirects to the http URL.
I'll make it to use https then, thanks for your feedback!

[takac]

LGTM. No issues here on 5.0.2 on Ubuntu.

[xuhdev]

There is one more thing -- would think ddg is better to be used rather than duckduckgo? But I don't know possible conflicts.

[mcornella]

I don't know, I left it like I found it. Notice that, as it is, it's backwards compatible with previously defined aliases. That said, we could add a ddg key and keep the duckduckgo as well, but I'd like some consensus on that before I do anything.