smart path configuration variables

[jkeung]

Request for smart path for configuration variables. Now currently only accepts the absolute path.

[bbengfort]

Create a path_setting helper function, similar to the environ_setting helper function.

This function will do the following:

1. expand ~ and the user directory symbol
2. expand envvars
3. norm the path (handling .. and . references)
4. provide an option to give an absolute path
5. provide an option to create directories if they don't exist

Todo:

• implement function
• write tests
• document