use the svginject javascript library

[aappling-usgs]

problems with getting twitter to pick up a tweet/card can be partly addressed by using the svginject javascript library, which would reduce page size. the library tells the client that svg is there, without actually loading it until needed

[jiwalker-usgs]

svg-inject allows a png fallback, we may want to consider supporting that.

[aappling-usgs]

all implemented on @jiwalker-usgs 's computer, just needs testing

[aappling-usgs]

need laura's PR merged for the example to build, need alison's PR merged to check for conflicts

[jiwalker-usgs]

#218 changes how SVGs work