Support attachment fields in htmlWidgets

[michaelquinn32]

Although not commonly used, htmltools::htmlDependency() has an attachment argument.
https://github.com/rstudio/htmltools/blob/master/R/html_dependency.R

When rendering the widget, this creates <link /> elements in the html.
https://github.com/rstudio/htmltools/blob/9abc3f2d9e23e9f02a0945c526d8a3de77f9cec4/R/html_dependency.R#L64

From what I can tell, the current repr method for htmlwidgets doesn't support this argument.