Investigate to implement an interpolate option

[rockneurotiko]

Sometimes you don't want just the variable in your config value, maybe you want it to be interpolated in a string, for example:

config :my_app, 
    some_url: "http://${MY_HOST}/wat

Some ideas about how to implement this, let me know which one you think it's the best one:

• As an option:

{:system, "MY_HOST", interpolate: "http://${ENV}/wat"}

Pros:
Easy to implement
All the options of this library

Cons:
Not flexible, you can only use one env variable

• As on option but uses the value:

{:system, "http://${MY_HOST}/wat", interpolate: true}

Pros:
Easy to read

Cons:
Not as easy to implement.
It's the same as replace_env_vars.
Lose all the flexibility of the library

• Using the env name in a post process:

{:system, "MY_HOST", interpolate: "http://${MY_HOST}/wat"}

This would allow to do things like:

  path: {:system, "PATH"},
  url: {:system, "URL", interpolate: "http://${URL}/${PATH}"}

Pros:
Really flexible and can use all the features of the library

Cons:
We'll need to save all the env final values and do a second pass with all the interpolate after all the envs has been replated with their values, so, the performance of the library will suffer a little bit.

• Do not implement and just use replace_env_vars