pyramid scaffolding collides with jinja2 templating

When Pyramid scaffolding finds files with the suffix _templ, it inspects the file for variables such as {{project}}, {{package}}, and {{package_logger}} and replaces them. However, when using Jinja2 templates, this collides with standard variable replacement, and the outcome is that pcreate dies.

To solve this problem, I've added a function ``substitute_escaped_double_braces`` that takes values the following string: `"\{\{ mystring \}\}"` and exports `"{{ mystring }}"`. This is handy because it allows a developer to use other templating languages without generating collisions.