Escape ampersand to conform with HTML validation

Validation error: `& did not start a character reference. (& probably
should have been escaped as &amp;.)`.
Add: option to count directed or undirected edges

Previously we only displayed interactions originating from the census
block that the marker was contained within. However, if direction is not
important to us then we can also use the reverse edge counts to make the
interaction strength undirected.