Github flavored parsing of underscores

[igrmk]

Github parses underscores not the same way as asterisks if they are inside such_an_identifier
Just look at this repo https://github.com/igrmk/github-markdown-corner-cases
This PR is adding support for such parsing
Asterisks are still parsed the old way

The feature is enabled only if markdown_syntax_github is set to 1

[tpope]

A global option is a cop-out. What behaviors in the old implementation are you trying to preserve?

[igrmk]

The problem appears if you use identifiers with underscores in italics.

With current implementation you can toggle italics, bold and bold italics inside the word using underscores. However GitHub ignores underscores inside the word in this case. You can still use asterisks to do it. So some GitHub documents look totally out of sync if you use identifiers with underscores in italics.

Example:

1. _Italics_Italics_ => Italics_Italics (vim-markdown)
2. _Italics_Italics_ => Italics_Italics (GitHub)
3. *Italics*Normal*Italics* => ItalicsNormalItalics (asterisks work the same in both)

First example would cause the rest of the document to be rendered in italics if you use vim-markdown. This is because last underscore would start unclosed italics block.

[tpope]

First example would cause the rest of the document to be rendered in italics if you use vim-markdown.

Nobody wants a single _ to highlight the rest of the document in italics. This is not a valid reason to add a configuration option.

I'd say your changes should be the default, but they introduce regressions. Try this:

this, _that_, and these

It works in the current vim-markdown implementation. It works on GitHub. Your implementation highlights the rest of the file. This is probably fixable though, by using \w instead of \S in some cases.

[igrmk]

I've fixed regression and removed configuration option