add option to force insert line breaks to highlighting code.

[dindinw]

Add a option which allow force insert <br> into the <pre> of the syntax highlighting code in the case when CSS "write-space:pre" not work. The option is off by default.

enable the option, let the line breaks work in the case.

[liudongmiao]

+1

@adam-p And if you want to merge this, I'll add a Chinese translation for this patch.

[concefly]

I really think so. +1

[adam-p]

In what scenario does "write-space:pre" not work? I don't mind adding this, but I need to know why.

[dindinw]

@adam-p originally, the issue I had faced when I tried to use wechat's editor(https://mp.weixin.qq.com/) to publish my articles, which failed to handle the line-breaks of the syntax highlighted code lines. I can't remember the details and reason why"write-space:pre"not work in the case after a long period of time, Sorry for my bad memory. Anyway, force to replace '\n' to <br> fix the issue.
hope this helps
-Alex

[lonnyzhang423]

+1

[adam-p]

@dindinw Can you sign a CLA and add it to the PR, like is described here: https://github.com/adam-p/markdown-here/blob/master/CONTRIBUTING.md#contributor-license-agreement

I'm inclined to merge this PR (after all this time), but I also wonder if it wouldn't just be better to use <br> unconditionally, without an option. But I guess having the option basically allows it to be tested in the wild for a while first.

[dindinw]

@adam-p
I am not sure the fix still work after so long time, I fix the conflict without testing anyway, hope it works.