Give headsup that we can't really do anything unti we have a `db` object.

Previously we would fail completely when mongo was started with --nodb.
There aren't that many usecases for --nodb that we should bother
attempting some workaround for it, but its nice for those who do use it
that the shell won't refuse to startup now :)
Make it possible to configure syntax highlighting

Users can now define the COLOR_* variables to overwrite the colors we
use for syntax highlighting.
These variables can either be defined at the top of this file, or
preferably using a "config.js" file (which is (hopefully) read before
the shell reads this file).
Recommend making ~/mongodb.js a directory to modularise mongo-hacker

When ~/mongodb.js is a directory the shell will actually iterate over
the content of the directory and load all the files one by one.
By recommending a naming convention "config.js", "init.js" and "my.js"
we actually make things quite modular as these files are read
alphabetically by the server.
This means we can define all configuration variables in config.js, keep
mongo-hacker as init.js (which uses the variables already loaded from
config.js), and then the user can add his own customizations,
enhancements or funcntions to my.js
Add a config.js template listing all currently-supported-variables