doc: add v2.1.0 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>
dag: rename --count to --max-count for consistency with git-log

Signed-off-by: David Aguilar <davvid@gmail.com>
dag: pass unknown arguments along to git-log

Use parse_known_args() and arrange for all unknown arguments to be
passed along to git-log via the dag query.  This allows any git-log
specific option to be passed to git-dag without its command-line parser
getting in the way.

Closes #389

Signed-off-by: David Aguilar <davvid@gmail.com>