Don't linkify the same portion of text twice

@rwaldron's fix of #96 doesn't solve the issue that overlapping portions of the text may be linkified more than once.  This fix also doesn't double-encode the &<> characters that are already encoded by the use of .textContent to insert the message into the DOM (linkify is applied to innerHTML, not to the message itself).

This is based against the commit before @rwaldron's PR, so his addition of the jsdom dependency would have to be reverted as well.