Line number support

I've added line number support for the issue https://github.com/Anviking/Chromatism/issues/15 .
ruddfawcett recommended the https://github.com/illyabusigin/CYRTextView project which already has a good line number support.
I implemented the class which is responsible for the line numbers (`CYRLayoutManager`).

Line numbers can be enabled or disabled with the `drawLineNumbers` property in the `JLTextView` class.
The Colors of the line number column at the left side can be modified using the `JLGutterBackgroundColor` and `JLGutterLineColor` key in a `JLTokenizerTheme`.