Check if the user has declared the plugin and dependency

[fasfsfgs]

As written on README.md from earlier versions, for this extension to function properly when using Connect to a running nREPL it's necessary for the user to add cider-nrepl plugin and tools.nrepl dependency.

If they don't do it, there are no clear warnings about it and the extension just doesn't work as intended (as seen on #30 comments).

The idea here is to check if those dependencies are correctly declared and warn the user otherwise.

[fasfsfgs]

I think a good start is to check for those dependencies when trying to connect to the repl. If we find out that the configuration is not done correctly, we warn the user and don't connect to the repl.

Agree @avli?

[avli]

This one will be automatically solved after #21 is done.