Search engine for projects which are possible to install with hell

Expected:

• added additional flag for hell --search -> -s
• added to .hell.json config additional(optional) filed which indicate hell to send to hell search server information about repo, when it is used in another one(during install or resolve step). If exist for given repository, then should contain info about it, which will be usable for users and search engine
• implemented search engine application deployed on server which should:
• allow users ask about known repos using for example: hell --search <regexp>
• allow repositories to be registered and have updated popularity during hell --install *

changed the description

Probably we need to reserve some "special targets" for current and future development of hell in .hell.json. For now I think that targets which start by @ are good for this purpose and can have special meaning for future versions of hell without backward compatibility break. @ have additional adventage in that it also have special meaning in cmake and cannot be used in "plain"variable name.

So I hope there can be introduced something like optional:

"@PUBLIC=": {
  "title": "libabc",
  "description": "This is abc which do xyz",
  "tags": ["first tag", .... "last tag"],
  "homepage": "https://abc.xyz"
}

And this will be first "special" hell target (except "=", which is not really special - rather conventional)

mentioned in merge request !8 (merged)