Support syntax highlighting

[ottok]

In 2017 the project https://github.com/gohxs/readline forked off this in order to provide syntax highlighting capabilities. That capability is the reason why some projects still depend on it, for example usql.

According to #116 the syntax highlighting capability was merged into this version.

Is anyone using it? Is it supposed to work?

In a quick test by @kenshaw this chzyer/readline does not seem to support syntax highlighting.

[ottok]

@chzyer Any comments on this?

[gamebox]

@ottok I think you can use a Painter to customize how the line is output. It gives you the current line and you can return whatever you want.