Change completion message.

Make it clearer it's showing the finish time (absolute clock time),
not the duration.
Command line flag changes:

1. Add `-s/--serve` command line flag.

`-s/--serve` runs the server, only (whereas `-p/--preview` runs the
server and also launches an external web browser).

The intended use of --serve is if you're running Frog remotely via
ssh, in which case to do a preview the web server needs to be there
but the browser would run on your local machine. Plus, the remote
server OS might not _have_ a GUI web browser --- or even if it does,
it would be silly to VNC to it when you could, um, use a web browser.

[Although I suppose you _could_ use --serve as the production server
for a blog, that's _not_ the intent. The intended use is just for
preview, and you will deploy the files to some other static web server
that's the real, production server.]

2. `-w/--watch` is now a modifier flag that must be specified in
addition to, and before, either `--serve` or `--preview`.

(Previously it meant watch-and-preview, by itself.)

3. Some flags may now be specified multiple times. Examples:

    # Create a couple markdown post source files:
    raco frog --new "Post 1" --new "Post 2"

    # Build, then preview with watch, then a final build
    # (because --watch currently doesn't guarantee a full build):
    raco frog --build --watch --preview --build