push arg-parsing into argparse

don't bother with sys.argv directly
add --port option
restore pick_a_port() behind --port, WEBDIFF_PORT

nice to allow users to not have:

1. any cfg set up and still get a port that works, and
2. not have to have 8080 free necessarily
Merge pull request #17 from ryan-williams/nits

Nits
Merge pull request #18 from ryan-williams/port

use argparse, add --port option
more specific error when directory-ness mismatches
die early when a file to be diffed doesn't exist
cfg option for skipping browser open on reload
default to not opening browser tabs on reloads
kill cfg option for browser reloading
Merge pull request #19 from ryan-williams/skip-browser-reload

cfg option for skipping browser open on reload
cleaner error msg printing
Merge pull request #22 from ryan-williams/errors

Die early when a file to be diffed doesn't exist
pass ORIGINAL_DIR down to reloader thread
Merge pull request #20 from ryan-williams/chdir

don't os.chdir to app.py directory
indent some html
remove unused var