Added markdown with atx style headers, can be chosen as
alternative markdown template on the cli.
The existing 'markdown' template will still default to
setext-style headers.
Formatters (previously templates) were pure data containers before,
continating the 'template' for how things should be formatted using
mustache. The formatting would be done a) in the exporters and b) in the
annotations.
This spread of formatting has now been consolidated into the Formatter,
which fixes the overall spread of formatting code and now can coherently
format a whole output instead of just individual annotations.
A formatter contains references to all documents and contained
annotations and will format everything at once by default, but the
formatting function can be invoked with reference to a specific
annotated document to only format that.
This commit should put more separation into the concerns of exporter and
formatter and made formatting a concern purely of the formatters and
annotation objects.
