Add worddiff.js to extract words from the HTML
Fix typos in the index

Found with worddiff.py
Write 1/2 using &frac12;, matching the typography of the book
Fix typos in chapter 3: Missions, Modes, and Manufacturing

Found with worddiff.py

multi-purpose is not hyphenated in chapter 10

178-(centi)meter is the most wrong typo so far...
Ignore white text in worddiff.js