Changed the linter gutter dot color from grey to something more noticeable

It took me a while to notice that lint errors show up in the gutter. I noticed someone else had the same issue in #524. This change makes the color of the gutter dot match the color of keywords from whatever color scheme you're using. In Monokai, it's fusia.

I wanted to make it red regardless of the theme but I haven't figured out where to add a scope with color information to the plugin.