Add interpolation to Config objects

[greg-1-anderson]

This PR interpolates values into tokens delimited by double-curley braces.

Example: $config->interpolate('This is a {{a.b.c}} bar');

This is a departure from how variables are defined and substituted at parsing time, where ${name} is used instead. It would be possible to be consistent, but consistency would make it harder to pull strings out of configuration and use them as interpolation templates. I'm not sure if this use-case is important or desirable.

[greg-1-anderson]

Also, I made this an interface / trait to make for easier calling, e.g. $siteAlias->interpolate(...). However, the downside to this implementation is that there's no way to call interpolate if you have an object that implements ConfigInterface but does not use the interpolate trait. Maybe I should make a utility class:

$interpolator = new InterpolateUtil();
$result = $interpolator->interpolate($siteAlias, $message);

Maybe I should do both.

[greg-1-anderson]

I did both.