Settings should be empty by default, user settings are generated
Bump the minimum queue delay a bit, it's annoying if it's too fast
Added support for errors types (warning/error).

- If the linter regex returns a 'type' of 'w', 'warn', or 'warning' (case-insensitive), the warning style is used. Otherwise the error style is used.
- Reworked Highlight/HighlightSet to support the error types.
- Gutter marks are drawn with separate regions to have better control over the icon and color. If a custom icon is used, the scope is set to a non-existent scope so it is not colorized.
- If a line has both an error and a warning, the gutter mark will be an error.
- Removed line outline support, it's just too intrusive.
Added executable attribute to linter class.

This allows the linter to be enabled/disabled based on the availability of the linter's executable, without overriding a method.

If set, it should be the plain name of the executable. If the executable is in the user's path, executable_path is set to the full path. If it can't be found, the linter is disabled.