usage + browser flag

[jbenet]

@davecheney thanks for making this!

this PR adds:

• usage output if no args given.
• ability to output the svg to stdout.
• browser flag to open in browser (currently defaults to false, which is TRTTD, but defaulting to true would match the current functionality, which is backwards compatible and doesn't break people's scripts).

> graphpkg
usage: graphpkg [flags] <package name>

> graphpkg -browser=true net
opening in your browser...

> graphpkg sort
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
 "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<!-- Generated by graphviz version 2.38.0 (20140413.2041)
 -->
<!-- Title: %3 Pages: 1 -->
<svg width="62pt" height="44pt"
 viewBox="0.00 0.00 62.00 44.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 40)">
<title>%3</title>
<polygon fill="white" stroke="none" points="-4,4 -4,-40 58,-40 58,4 -4,4"/>
<!-- N0 -->
<g id="node1" class="node"><title>N0</title>
<polygon fill="none" stroke="black" points="54,-36 0,-36 0,-0 54,-0 54,-36"/>
<text text-anchor="middle" x="27" y="-13.8" font-family="Times,serif" font-size="14.00">sort</text>
</g>
</g>
</svg>

:heart: @jbenet

[jbenet]

I ended up making other changes. I'm pushing to my master, so they'll also show up here (sorry, i should've made a feature branch). This PR was originally just for 6b839b7...360d2cc

Feel free to totally ignore them, they move this into a bit of a different direction. Look: http://bl.ocks.org/jbenet/c51cd7eb730d18e5cdc2

non interactive version: