Added option to stop messages being shown all the time (default behaviour unchanged)

I added a `g:reek_always_show` variable that when set to 0 will stop messages being shown on loads/reloads. The default behaviour is still to show the messages (as it was before this change).

When I'm working on other people's code, I'm not going to be able to get rid of all the warnings, so I like to be able to ignore them until I can do something about it.