D3 directed graph

This adds a util to create a self-contained html file with a directed graph of either the mentions or the retweets in a twarc archive. It works like the other utils (uses fileinput to read from stdin or from file list), with an optional arg "--mode" which may be "retweets" or "mentions" (default: retweets). The html is generated from a template in resources/directed.html, into which the script inserts the output json.

Lots of enhancement of the D3 presentation is possible, but this is a good start.