Fixing some italics syntax consistency issues

[ryankinderman]

• Added skipping escaped italics tokens when end matching
• Updated prefix/suffix constraints for start/end patterns to be more consistent with behavior of the Markdown Dingus
• Added constraint on start pattern so that it doesn't match if there isn't an end token within the same paragraph
• Added sample.md to syntax dir to use for testing syntax consistency w/the dingus

[RubenVerborgh]

Interesting work. However, I believe there are still some cases that are not covered. For example:

*half*-italic
# Title 1
*half-*italic
# Title 2

The second title does not get correct syntax highlighting, even not after patching.

[ryankinderman]

Yea, I'm sure there are cases that still aren't covered. I basically tried to fix the handful of cases that were not working for me at the time. It's hard to be comprehensive with nothing to use as a reference except the "Dingus". Would love to see others jump in and fix things that are still missing.

[silverhook]

I tried it and I get a similar bug as @RubenVerborgh, but slightly different.

In his example, the Title doesn't get highlighted. In my case just every _ after the first # seems to be shown as a syntax error.

The bug I describe here is not present in @tpope's current master