Allow creating files with extensions

[mike-lloyd03]

This adds support for creating files with predefined extensions. This allows temporary files to be created for opening in text editors that recognize filetypes (e.g. vim or neovim). See #13.

Four new public functions are added:

• new_file_with_extension
• new_file_with_extension_in
• new_path_with_extension
• new_path_with_extension_in

I've also tweaked the create_path* functions to allow for this functionality and added tests.

[mike-lloyd03]

@samgiles Any chance you could take a look at this?