Move to latest version of Scalasti / StringTemplate V4

This patch allows `giter8` to use the latest version of [Scalasti](http://software.clapper.org/scalasti) based on [StringTemplateV4](https://github.com/antlr/stringtemplate4) so it replaces #77

In StringTemplateV4, you configure the renderer on the [group](http://www.stringtemplate.org/api/org/stringtemplate/v4/STGroup.html), not the [template](http://www.stringtemplate.org/api/index.html?org/stringtemplate/v4/ST.html). Unfortunately Scalasti only allows you to use [file templates with groups](https://github.com/butlermh/scalasti/blob/master/src/main/scala/org/clapper/scalasti/STGroup.scala#L124), not string templates, so it was necessary to do some monkey patching to call the underlying StringTemplate API. 

This also means that there is the potential to modify the template delimiters #132 although unfortunately StringTemplate has the limitation that it can only cope with single characters.