String concatenation and interpolation

[bocchino]

It would be useful to be able to concatenate strings and interpolate string constants into other strings. This would help with configuring subtopologies. For example, one could provide a configuration constant

fooConfig="defaultFooConfig"

and then provide a subtopology code snippet like

"configureFoo(" + fooConfig + ")"

or

"configureFoo($fooConfig)"

Then the user can configure the code snippet by providing a new definition for fooConfig.