Add configuration option to disable the default hidelines feature for rust

[zgtm]

Simple change that adds a configuration option to disable the default hidelines feature for rust:

Usage:

[output.html.code]
default-hidelines = false

I need this in my book since I have several multi-line strings that have lines starting with #. (Embedded code snippets with preprocessor directives.)

Let me know if this needs a test.

If you want I can also add a line to the manual, but since this is a niche feature, the doc string might just be enough. :)

[Edit: Added a test]