Add mls-mode-line-string

Add a new variable `mls-mode-line-string` to insert into the `mode-line-format` instead of custom calls for each module (should fix Issue #14).

Use [dash.el](https://github.com/magnars/dash.el) library in `mode-line-stats.el` to make code more readable.
Ah! nice catch!
I have changed that lines a few times and I missed that.
I'm not sure what will happen if the same module is added/loaded 2 times. But I think it makes more sense that a module is only load once.