Add default values handling into string substitutor

[groov1kk]

Hello !

That was great that you've added placeholders into configuration path resolver.

But how about adding the ability to set the default values for specific placeholders, if corresponding properties are not specified?

For example ${property:defaultValue}

Just like Spring's placeholder configurer supports.

[coveralls]

Coverage remained the same at 92.961% when pulling e184c8b on groov1kk:default-property-placeholders-values into a2003f6 on lviggiano:master.