Customizable start/end symbol for template fields

Thanks for developing and maintaining `giter8`.

I couldn't find anything in the documentation about this, but is there a simple way to use a different symbol for identifying template fields?

It looks as if `$` is hard-coded in several places:
- [`Template.scala`](https://github.com/foundweekends/giter8/blob/f96eaeed130a08892fe8dcc7e9d035dbe13e92ad/library/src/main/scala/giter8/Template.scala#L34)
- [`StringRender.scala`](https://github.com/foundweekends/giter8/blob/f96eaeed130a08892fe8dcc7e9d035dbe13e92ad/library/src/main/scala/giter8/StringRenderer.scala#L27)

Instead of using `$` for `$var$`, I would like to be able to specify a different start/end symbol (ex. `%$%` in `%$%var%$%`). It would be nice if this could be handled via a configuration property.

Perhaps this is a way to address string interpolation conflicts described in #102, #288, and elsewhere?