Suggest knitr and create simple md_table() option

[k5cents]

Instead of simply wrapping around knitr::lable(), we could create a very simple alternative and then only use the superior knitr::kable() when the user has it installed.

[k5cents]

The fallback table method is absolutely terrible and requires a lot of unnecessary dependencies, which kind of defeats the purpose of only suggesting knitr::kable(). There might be a way to do it with more basic base R, but it might be more trouble than it's worth.