opts_template: Template for creating reusable chunk options
Description
Creates a template binding a label to a set of chunk options. Every chunk
that references the template label will have the specified set of options
applied to it.
opts_template$set(myfigures = list(fig.height = 4, fig.width = 4))
# later you can reuse these chunk options by 'opts.label', e.g.# <>=# the above is equivalent to <>=