argparse configuration file

This is an attempt to let a configuration file provide defaults for argparse comand line options, using Python's `ConfigFileParser`

Main points:
* Config file values are automatically cast if a `type` is specified in `add_argument`
  * Limited support for the `action` parameter of `add_argument`: `store_true`/`store_false`/`count` will result in a config value being automatically cast to `bool`/`bool`/`int`, other actions are untested
* I've tried to avoid referencing any private underscore classes from `argparse` (though this makes things more complicated)
* Parsing has to be done in two steps:
  1. Look for any config file arguments, and parse those files
  2. Parse the remaining arguments
* Parsers/subparsers can reference one or more sections from ConfigFileParser
  * Multiple sections can be specified and merged (e.g. you could have a defaults section overriden later)
* Shouldn't require significant changes to users of this class.
* Configuration keys are named after the argparse `dest` value (e.g. `dry_run` not `dry-run`)
* Note added to README.rst

Corresponding omego PR/config: ome/omego#47
See also ome/omego#41
I'd have to look more closely. There are of course all manner of hacks we could use until a breaking release:
 * a global registry of config files
 * assigning a field externally
 * ...