add an inline argument to textOutput(), imageOutput(), plotOutput(), and htmlOutput()
closes #501: knit_print() for inline output uses the `inline` argument passed from knitr
preserve the formal arguments of plotOutput() in outputFunc()

otherwise outputFunc() loses the `inline` argument, and knit_print() won't be able to produce inline plots
roxygenize
merge doc of width and height in renderPlot() and plotOutput()
ignore width/height in plotOutput() when inline=TRUE, and document required width/height values in renderPlot()
roxygenize
Merge pull request #512 from yihui/feature/inline-output

Inline output in R Markdown documents