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/1850538/4112614/3c9ddf6a-3232-11e4-96d3-295c91584507.gif)

This is fine, we're storing in the DOM anyway so no need to store it twice.