Support EL in Config value

[hantsy]

For example, in a config properties file, we could use EL as partial value of config property.

a.name=hello
b.name=#{a.name} world // or ${a.name} world

[radcortez]

Yes. I think we all agreed that we want to support some expression replacement mechanism.

I don't know if it should be EL or something else. Probably, EL has a lot of things that do not make sense in this context, so we need to have a proper discussion around it.

[radcortez]

Duplicate of #77