Nested list highlight broken for tabs

[rishabh-ranjan]

nested lists are highlighted correctly for 4 spaces indent, but not for tab indent:

[alerque]

Feel free to submit a PR. Also be aware that not all Markdown implementations support tab sublist indentation and also even fewer of them accept mixed-mode indentation. But yes tabs are usually considered the equivalent of 4 spaces for about 3/4 of implementations. c.f BabelMarkIII

[rishabh-ranjan]

in that case, maybe it will be better to set tab to 4 spaces via vim markdown. this would be as simple as adding "setl tabstop=4 shiftwidth=4 expandtab" in indent/markdown.vim. @alerque if you approve i can submit a pr for this