The third_party svg directory has been a mix of pprof-specific code and
svgpan itself. This change moves the pprof-specific code out and adds a
license file. It also updates svgpan to 1.2.2 from
https://github.com/aleofreddi/svgpan while we are here. It also renames
the directory to svgpan to have all directories under third_party be
named precisely over the third party package they hold.

For certain formats (eog, evince, gv, web, weblist), the visualizer
would be invoked regardless of whether the user specified a specific
output file to write to. In order to ensure that output is properly
written, the invokeVisualizer function has a check for whether the
output is os.Stdout and alters behavior based on that. This is the
wrong abstraction layer to do that work.

At the core of the problem is that PostProcess is used to both
do data post-processing that is inherent to the format, and also
to invoke some visualization for the data (to Stdout or browser).
We split these two steps apart and make it obvious which is which
by adding a visualizer field to Command.

This seperation of concerns allows us to simplify the code.

Also update the build instructions to use go get