Library to read configuration values from edn resources,
environment variables, and Java system properties.

Values are loaded in this order, with later values overriding
previous values:

 - `/resources/conf/base.edn`
 - `/resources/conf/default.edn`
 - `/resources/conf/defaults/$CURRENT.edn`
 - `/resources/conf/$CURRENT.edn`
 - Environment variables
 - Java properties

The current environment is determined by the special environment
variable, the 'env-key', `CONF_ENV` (or the Java property
`conf.env`). The value of the env-key determines the name of
additional edn resources to load.  First
`/resources/conf/defaults/$CURRENT.edn`is loaded and then
`/resources/conf/$CURRENT.edn` is loaded (if they exist).  Values
set in these files will take precedence over those set in the
previous configs. For example, to run a program in a production
configuration, you might run the command: `CONF_ENV=prod lein
run`. This would load the base config file,
`/resources/conf/defaults/prod.edn`, and `/resources/conf/prod.edn`,
with the latter files taking precedence over the earlier ones. Note
that the value of `CONF_ENV` can be an arbitrary string; you can
name the environment-specific config resources however you like.

As listed above, environment variables are also merged into the
config. These take a higher precedence and can override any file's
value. Names of environment variables are normalized from
UPPER_UNDERSCORE_CASE strings to more Clojure-esque lower-dash-case
keywords.

Finally, Java system properties are also merged into the
config. These take the highest precedence of all, and can override
any other config value.  Property names are normalized from
lower.dot.case to the typical lower-dash-case, just like environment
variables.

If for some reason you don't like the default env-key, `CONF_ENV`, you're free
to choose your own by calling `load!` with an additional argument.