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)

Can we use a highlight color from the theme? E.g. dark-ui-them has this `@text-color-highlight` variable: https://github.com/atom/atom-dark-ui/blob/master/stylesheets/ui-variables.less#L5

Using that should make this look nice with any theme.