add +__if_render.me__+ statement for conditional rendering #29 enhancement "Conditionally render files based on answer.

Variables can also influence rendering. Surround variables with plus signs. 
For example `foo/+__if_render.me__++author+/+age+.bob` given variables *author* being `Foo` and *age* being `12`, `foo/Foo/12` will be rendered if `render.me` is True. 

 Else only `foo/` will be rendered. Please notice that only ('y', 'yes', 'true', 'True', 1) are True, anything else will be considred as False.

it's a proposition for #29 enhancement "Conditionally render files based on answer.

I added tests and of course coverage is 100%. Documentation must also be reviewed, for a more clear comprehension.