wish: improve default experience

[markstos]

I ran the script with no arguments just see if the requirements were met and it didn't blow up.
Here's what happened:

$ python ./py-chrome-bookmarks.py
Traceback (most recent call last):
File "./py-chrome-bookmarks.py", line 56, in
in_file = os.path.expanduser(sys.argv[1])
IndexError: list index out of range

A more user-friendly default behavior would be nice, perhaps remind the user of the expecting calling convention and exit.

[bdesham]

Fixed in 1.1.