6.0.0: Block params, else if, log levels

• Changed log helper to use level="..." argument to control log level,
  and its default to use loglevel info, aligning it with HandlebarsJS.
  See https://handlebarsjs.com/guide/builtin-helpers.html#log
  (@thekid)
• Merged PR #17: Add support for else if as syntactic sugar for nested
  if / else cascades
  (@thekid)
• Merged PR #16: Add support for each with block params. This pull request
  also includes a more efficient iteration algorithm as a side-effect.
  (@thekid)
• Made compatible with xp-forge/mustache version 7.0, which emits single
  quotes as '.
  (@thekid)