Add handle to multi platform plugin install

[otherpirate]

Add support to multi platform pluging install

Check list

• [x] Parse JSON fields
• [x] Install multi platforms
• [x] Update tests

Closes #172

[otherpirate]

@tcarreira could you please review it?

[otherpirate]

Great description @tcarreira, thank you 👏

I hope now we are ready to go

If there is other concerns, please let me know

[otherpirate]

Giving up, thx @tcarreira

[tcarreira]

nooo, don't give up @otherpirate 😉 If you activate the "Allow edits by maintainers" I could help you finish it.

Of course, if you still want to give up, it's up to you.

[tcarreira]

Hey @otherpirate After the merge of #191, this PR can be implemented. Do you want to give it a last try?

what should be done:

• changes go inside func installPlugin(pluginName, pluginURL string) error
• nothing should be changed until http.Get(pluginURL) (actually, until defer resp.Body.Close() referring to that line)
• bind the content of the pluginURL to the new type PluginManifest. If it fails, keep the old behavior
• get GOOS and GOARCH from runtime
• get the new URL from that PluginManifest.UrlPerPlatform[GOOS/GOARCH]
• recursively call installPlugin() with this new pluginURL

To prevent infinite recursion, the func installPlugin(...) could take an extra argument: a counter to be checked for recursion

Let me know either decision ;)