Add optional feature: alternating line colors

[v972501]

Hello. I really like micro editor and wanted to add this feature. It alternates line colors like in old printer terminals or like excel spreadsheets - it makes navigation easier, especially with long lines:

Code introduces a new bool option: altlinecolors, it's set to false by default. If it's set to true and alt-line color is given in the colorscheme, then it would change every other line's bg color to a specified one. I've included a demo colorscheme as well. I also posted a video demo in the Gitter group.

[dmaluka]

TL;DR I believe this code is messy and buggy. Please see my below descriptions of the bugs I've found. But instead of addressing them individually, I suggest to simply replace this implementation with dmaluka@18d9c5c (which not only simplifies the code but also makes these bugs go away).

[v972501]

Great refactor, thank you! Can I replace my changes with it in this pull request? (I'm new to github, sorry if it's silly question)

[dmaluka]

Yes, you can use git push --force. (I'm assuming you are using git, not just github UI?)

[v972501]

ok, replaced with proposed refactor.

[v972501]

I've added alt-line colors to some runtime colorschemes.
here's preview of non-tc ones with MICRO_TRUECOLOR=0 COLORTERM=24bit env:

-tc ones with MICRO_TRUECOLOR=1 COLORTERM=truecolor env:

[dmaluka]

To me personally, in your truecolor colorschemes, the difference between alt-line and normal line colors is barely noticeable (unless I look closely). Is it just me?

[v972501]

To me personally, in your truecolor colorschemes, the difference between alt-line and normal line colors is barely noticeable (unless I look closely). Is it just me?

I tried making them brighter, but they would become very similar to cursor-line color.
Making them darker than default lines didn't look very good.