Make template interpolation configurable in @sakuli/commons

[tnobody]

Short overview

@sakuli/commons provides a handy template rendering function to which currently hardcoded interpolates values which are escaped by ${...} to interpolate values from other properties #51.

Use case

Reuse Template functions for other interpolation scenarios than ${...}

Detailed description

We will introduce a second parameter which allows the manual setting of an interpolation RegEx, which has the current interpolation as default value.