Add option to show all errors in status bar

I find it useful to have the possibility to display all errors and click on the error to go to the correct line/col.

Previous PR: https://github.com/AtomLinter/Linter/pull/181

![atom-show-all-errors](https://cloud.githubusercontent.com/assets/1850538/4122756/dd803b04-32c7-11e4-90d4-de83a0f26196.gif)


Good catch. 

Since your code is executing in the same context as this existing code, it's probably safest if you copy it. How about factoring that code out into a helper method that can be used in both places.

(For the record, I didn't write that code so I'm not sure why it's done like that. In fact I don't like this whole approach of grabbing the editor from a global.)