Add the method 'render_file' for rendering a file

render_file takes two arguments, the first one is the file name that Rcurse should read its content
and the second one is the out file that rendered content will be written into.
Also, the method 'render' should only be use for rendering a content, not a file.
Merge pull request #1 from s1n4/render-file

Add the method 'render_file' for rendering a file