Add support for global config file snitch.yaml

[pannapudi]

At this moment, snitch requires a config file in the root project folder to add keywords. It would be convenient to have a global keywords for all projects and not to create snitch.yaml every time.

[rexim]

@pudnax Configuration is not only for keywords. It's also for title transformations: https://github.com/tsoding/snitch/tree/02dbeb923b1a625dbd0ae0489ba02300642b1b13#issue-title-transformation

Title transformations usually are the same for a lot of projects because they solve the problem of removing trailing "garbage" of multi-line comments like /* TODO: text of the todo */ where the "garbage" is that */ at the end.

With the global config I'd like to be able to put all of the transformations for different styles of comments to the global config so I don't have to add them to the local one. So I can keep only custom keywords in the local configs.

So the global and local configs should be "merged" somehow to be actually useful in the majority of scenarios. Tbh, I don't really know how exactly they have to be "merged". What do you think?