Support for end-before and start-after options

[UncleGoogle]

... that are present in standard .. include directive.

[dmaljovec]

@UncleGoogle I took a crack at solving this issue as I was running into a similar problem wanting to only include portions of a markdown file elsewhere in my project. Let me know if this solves your problem, too, or if you have further suggestions on my PR.

[UncleGoogle]

Nice! I will test it in a week. As far as unit tests are concerned, IMO the best and simplest option would be to compare results of mdinclude and literalinclude for different config. But it's just an opinion.