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)

I don't like this label. How about a copy icon as used in GitHub?

![image](https://cloud.githubusercontent.com/assets/4427/4115646/c6e61492-327a-11e4-8376-3fd9bcd6f1d0.png)

Another problem I have with this is that being at the end of the line means it's not lined up across multiple lines. I'd like to either right-align it, or put it on the left (perhaps before the linter name?).

Another thing you could do to make this look better is hide the icon by default, and show it only upon hover. That way the view won't be cluttered by a lot of repetitive copy icons.