Fix notes never rendering in text output

[itspriddle]

Relates to #21

The default event template never referenced {{notes}}, so show-notes = true and --hide notes had no effect on text output. Notes only appeared in JSON.

This adds a Notes section to the default template, after Attendees and before Recurs. It's still off by default (show-notes = false).

• Multi-line notes are indented under the Notes: label via a new {{{indentedNotes}}} template variable. Both LF and CRLF line endings work, and blank lines get no trailing whitespace.
• Notes are rendered unescaped, so <b> is not turned into <b>.
• truncate-notes still applies.

Example with show-notes = true:

    Notes:
      <b>Booked by</b>
      Josh Priddle
      josh@josh.fail

Not in this PR: the --hide recurrence / show-recurrence toggle and stripping HTML from notes.