Port gutter mark icon themes support from SublimeLinter

There was a mention of support for PNG gutter mark icons in readme. I like the icons used in SublimeLinter, so I ported them to Anaconda.

I have added another option (`anaconda_gutter_theme`) that is only used if `anaconda_gutter_marks` is set to `true`. If no theme is specified and gutter marks is turned on, everything should behave as it did before this commit.