Prevent options from becoming global.

The pie plugin was a little too clever in its use of closures.  In
processDatapoints it set canvas, target, and options for use in other
functions.  Since options was not declared this meant that it became
global.  Pages containing multiple pie plots therefore saw a range of
weird effects resulting from earlier plots receiving some of the options
set by later ones.  Resolves #1128, resolves #1073, resolves #1055.