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)


Before, there were 1call to show() per error so it created a lot of event listener if there were a lot of errors.

In fact, we just need one call to this function if there is 1error or more.

`̀``cofffeescript
# before - call show for every error
    for item, index in messages
      ...
      @show()

# after - call show if there is a violation
    for item, index in messages
      ...
    if @violations
      @show()
      @highlightLines()
```