number of clean up and housekeeping tasks

[twobraids]

this represents a number of tasks that clean up of various issues.

1. prepare to support multiple command_line value sources. That means rework where we decide to do help
2. add the ability to use a "$" suffix on an option to signal that the option is not to be written out to a configuration file. This will enable the future argparse support to have multiple arguments with the same name.
3. move fetching value sources out the inner most loop in the overlay/expand loops. That is much more efficient.
4. several formatting cleanups

[twobraids]

@peterbe I've broken the argparse PR into two parts. This part makes some relatively minor changes to the core of configman. In the grand scheme of separating implementations, the next PR will only deal with adding files that support argparse and their registration in the configman core.

r?

[twobraids]

hmmm, i thought we had already merged the change that made the form of all the imports the same. We discussed a month or two ago about when to move to the multiline import form and when not to. I find that consistency is more readable than having two different forms of imports.

[peterbe]

Nothing obvious sticking out except some low-hanging nits. I struggle to keep up with this whole argparse side of things since I'm not using or depending on it so my grey matters don't get the exercise.