Adopt cobra

[yuryu]

Cobra seems like a popular package used for CLI applications.

Pros:

• Single command for all features (like server and collector)
• Extensible
• Go community seems pretty familiar with the syntax
• Reuse the same code for options and configurations handling

Cons:

• Breaks compatibility because we'll remove collector

After cobra, it'd look like:

• open_saves serve to run the server
• open_saves collect to run the garbage collector

We could also add commands like restart, stop, reload, etc in the future.