Plugin to optimize path data by removing small details

[behdad]

See for example:

http://bost.ocks.org/mike/simplify/
http://stackoverflow.com/questions/13730062/simplifying-svg-path-strings-by-reducing-number-of-nodes

We need to be smarter with curves.

I need this, to optimize flag SVGs in https://github.com/behdad/region-flags/tree/master/svg
Currently some of the flags are over 200kb. That's a bit on the excessive side. :)

cc @jamesgk

[GreLI]

See #411

[behdad]

Awesome. Lets follow up there.