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.
`add-to-list` already does a `setq` :) Also, it checks if the list already contains a value to be added in which case it skipps adding it. Do we care about duplicates here? It could cause some troubles if two modules by chance return the same value from `mls-run-module`.