Feat/show always

[Servostar]

Added option show_always which, when set to true will always show the scroll bar even if the buffer can be displayed at once.
By default set to false.

(Should have) close(d) issue #73

[Julian]

This sort of thing would be very useful to lean.nvim, where we show things that don't appear at all in the sign column (something like LSP processing status for lines, see the yellow bars in the screenshot).

Screenshot

In our case though I tihnk this as-is doesn't go "far enough", in the sense that I want my satellite handler to imply always showing the scroll bar without the user having to change a setting (though of course they still should be able to turn things off). That may be different enough that it's separate from this PR, but going to have a closer look to be sure. Mentioning in case that's at all relevant to you as well @Servostar.