Offer code mark styles and gutter marks

_From @darkred on July 10, 2016 23:30_ _(Using VSCode 1.3.0 in win 10 x64 build 10586 with vscode-eslint extension 1.10.18)._ Currently the code marks(for errors and warnings) appear only as squiggly underline (i.e. as **spellchecker errors** in browsers, e.g. Firefox, Chrome): _(the error is in red, and the warning is in green)_  My suggestion is to offer more code mark styles (ideally the **outline** style, as it highlights the errors/warnings the best way -see below- ) and **gutter marks** : _(with the errors to be in red, and the warnings to be in yellow, as ESLint itself uses these colors - not green)_  ([here is another screenshot](https://sublimelinter.readthedocs.io/en/latest/_images/marks-outline-dark.png)) It's more easy to distinguish the errors this way. Thank you --- _For reference SublimeLinter3 package offers [five different code mark styles](https://sublimelinter.readthedocs.io/en/latest/mark_styles.html) :_ - fill, - outline _(the default style)_, - solid underline, - squiggly underline, and - stippled underline _and two types of gutter marks:_ - errors, and - warnings _Copied from original issue: Microsoft/vscode-eslint#99_