Add support for a --query option (along with a default configuration)

Since I don't like programs that require configuration, here we make
this script self-configuring. When run with no options, it will print
an example usage for a simple query:

	notmuch-to-html --query=tag:inbox

Then, if run with that query, it will use a very simple, built-in
configuration to run the query. Finally, it will also print out,
(to stderr, to avoid breaking redirection), a sample configuration
file so that the user can customize the output.