Alpha Release 0.1.8

Date: 2020-05-28

• Fix 'normal' number formatting.
• Fix scientific and engineering number formatting.
• NumberFormat enum is now string-valued to reduce confusion.
• Unit's now get the same .html/.tex protocol as expressions.
  • Data objects like Vectors will get this soon.
• StyleContext now proxies the .set and .wrap methods, allowing easier local control.
• Style keys are now validated. No more freestyle styles.
• Only try to load katex once. If it refuses, move on.
• Add missing 'serve' package dev dependency.
• Update to Typescript 3.9.3.
• Add the ObservableHQ test page for the package.
• Use \Leftarrow to mark function units
• Improve various formatting
• Add piecewise functions.
• Try to avoid redundant CI build on release.