Inline knit_print()

[yihui]

knit_print() can have an optional argument inline = TRUE/FALSE. When an input or output is to be rendered inline, we'd better not use div, but use span as the top-level container. One way to tackle this problem is to modify the tag object when inline = TRUE, but I guess this may not work for all inputs/outputs, as some still contain div's inside the tag. The other way is to add a CSS class to all tags, and define this class as display: inline, but this may break the positioning of some elements. Or we just give up certain inputs/outputs, and render inline tags only when it is easy and appropriate to do so.