theme_animint(rowspan, colspan, last_in_row)

[tdhock]

closes #115

this proposal supersedes #139 which should be closed if this PR is sufficient.

The idea is to define three new plot attributes, to be specified inside theme_animint

• rowspan=2 means plot takes up two rows (default=1)
• colspan=2 means plot takes up two columns (default=1)
• last_in_row=TRUE means this plot ends a row, HTML <tr> element. (default=FALSE)

the R code compiler needs to save these as attributes of the corresponding plot list.

For example

animint(
  ggplot()+theme_animint(rowspan=2)+ggtitle("first td in first tr, rowspan=2"),
  ggplot()+theme_animint(last_in_row=TRUE)+ggtitle("second td in first tr"),
  ggplot()+ggtitle("first td in second tr")
)

should make HTML like below

if any of these three attributes are specified in any of the ggplots, then the renderer JavaScript code puts the plot in the corresponding tr and td of the "outer table," according to the following rules:

• we create a new <tr> before the first plot, and after every plot with last_in_row=TRUE
• for every plot, we create a new <td> inside the current <tr>

There is no need for computing the total/max number of rows/columns.

@siddhesh195 please consider this PR and if you agree, close the other PR, and move any relevant code to this branch (compiler R code, tests, etc).