Implement footnotes for LaTeX output

[yihui]

Currently LaTeX output for footnotes is not implemented:

cmark-gfm/src/latex.c

Lines 447 to 449 in c32ef78

	case CMARK_NODE_FOOTNOTE_DEFINITION:
	case CMARK_NODE_FOOTNOTE_REFERENCE:
	// TODO

I don't have much expertise on C but I wonder if the team is interested in getting it implemented. Basically what we need to do is to add the footnote content to \footnote{} in the case of CMARK_NODE_FOOTNOTE_REFERENCE, and output nil in the case of CMARK_NODE_FOOTNOTE_DEFINITION .

  case CMARK_NODE_FOOTNOTE_DEFINITION:
    break;
  case CMARK_NODE_FOOTNOTE_REFERENCE:
    if (entering) {
      LIT("\\footnote{");
      // How can I get the footnote content here?
    } else {
      LIT("}");
    }
    break;

Thanks!