Subsections will appear underneath their top-level section, in
the order defined in the config file. They will use a ^ or ~
underline rather than -.
Users define subsections by adding a number to the end
of the section entry, either 2 or 3. It will be a subsection
of the parent section above it. This syntax is simpler than others
considered, like nesting lists inside a top-level section. It also
makes it easy for us to support deeper levels of nesting if we'd like.
Like normal sections, in users' notes files, they will use the
ID they define for each subsection.
Change-Id: Ib7b4781de81e19d11a7acb863204067c0946827b
Story: 2010375
ee73c9409e