Macro syntax in template path for accessing built-in templates

[NQNStudios]

When running fractive from the command line via a global npm installation, it becomes very difficult to reference one of the example templates in an original story without copying the file to a new location relative to the story, not the fractive installation.

I'm thinking we could implement a hackish macro for that like letting the user write the path $(builtin)/basic.html in fractive.json and Compiler.ts would interpret that into the path of the fractive installation.

[NQNStudios]

Note: at compile time, the global __dirname variable contains the path of the current source file (i.e. fractive/lib. We can use this to access the location of the fractive installation.

[NQNStudios]

Closed by #49.