Intra-word underscores need proper support

[alerque]

PR #269 attempted to fix this, but we had to revert the change (14c778e) because it caused other more serious problems (see #285).

At some point an implementation that doesn't break everything would be great!

[fmoralesc]

At some point an implementation that doesn't break everything would be great!

Yep.

[rwxrob]

Intraword underscores are specifically not a part of the Pandoc Markdown standard, which suggests using intraword stars for emphasis, which also is incorrect in this plugin (an inadvertently caused many of my students to misinterpret the standard because it looks so ugly even when done properly). I recommend to them not to use intraword emphasis at all because it makes their whole page colored incorrectly, which is a shame really because a lot of work went into making that work with Pandoc.

[p3palazzo]

Related to this is underscores in link URLs, for example https://commons.wikimedia.org/wiki/File:Wooden_roof_structure,_132,_ubt.JPG. Because some underscores are placed between a letter/figure and a punctuation mark, they are not technically intraword, but should nevertheless not trigger vim-pandoc-syntax to treat them as the start of an italicized string, as they do now.