patman: Implement the series subcommand

patman: Implement the series subcommand

Add a command to support management of series. These are local branches
which may have been sent to the mailing list, so may appear on the
patchwork server.

The new functionality uses a database, stored in a local file.

Various operations are supported:
   - add a new series
   - send a series and automatically link it with patchwork
   - 'increment' the version, to prepare to send an updated series
   - gather review/test tags from patchwork
   - check on progress, i.e. patches which are received new tags or
     comments
   - show comments on patches and cover letters

Signed-off-by: Simon Glass <sjg@chromium.org>