Return exit-code 1 when suggestions are found

Combined with jonase/lein-kibit#3, Kibit will exit with exit-code 1 when suggestions are found. This is most useful for CI tools.

Still to come before this is ready:

- [x] Check this was done idiomatically, need to think about the `for`
- [x] We might need to make a new `lein-kibit` function to call which then calls run, as `run` is calling System/exit at the moment which may break other usages of `run`.
- [x] Check behaviour on files passed directly
- [x] Check memory usage, as we are now holding onto all of the suggestion results

Fixes #88, and fixes jonase/lein-kibit#2