Introduce Rubocop

[TomasBarry]

When contributing to the Slacked project, there are no checks on code styles etc. I propose introducing Rubocop, and potentially HoundCI so that there are consistent coding guidelines and checks to ensure that this project maintains well written Ruby code.

I have experience with both Rubocop and Hound and have introduced both in a number of projects and would be willing to do so with Slacked if the maintainers see this as a valuable addition?

[kaiomagalhaes]

Hey @TomasBarry, both Robocop and HoundCI would be great additions to the project. Please submit the PR if you have a chance.