This does multiple things to "improve" tmap2svg

1. add a step to add a non-transformed g for svgPanZoom to use as its target viewport. Now, the pan works as expected.
2. adds height and width parameters to tmap2svg that allows height and width separate from tmap while retaining the same aspect ratio.
3. removes some of the SVG elements that don't work well in the htmlwidget context, such as the rect surrounding the map.

Note: need the dev/0.3.0 branch of `svgPanZoom, so

devtools::install_github("timelyportfolio/svgPanZoom@dev/0.3.0")

Here is a working example http://bl.ocks.org/timelyportfolio/e2effb970b3cbe2f4e84.