Load plugins from a given URL

[johnSchnake]

Describe the solution you'd like
When someone specifies the --plugin flag it can currently be:

• a known word like e2e or systemd-logs
• a name of a local file which holds the plugin definition

I think a 3rd option should be supported to aid in sharing/experimenting with plugins: loading from a URL.

Anything else you would like to add:
I think this is pretty trivial to do; we just need to check the string and see if it parses as a URL. If it does, do a GET to the URL and the response body is the io.Reader that we can pass around.

This would not be the most secure feature since the user could be targeting a URL they haven't verified the contents of or they could be subject to MITM attacks but I think this is still a useful, feature to make experimentation/sharing more simple.