WIP: Allow non-strings to be used for interpolation

[WeatherGod]

• Have the interpolation engine always use the string representation
  of a value when recursively processing the interpolation.

Closes #129
Closes #156

[coveralls]

Coverage increased (+0.04%) to 81.503% when pulling e4d0e46 on WeatherGod:interpolate_non_strings into 5b5de48 on DiffSK:master.

[WeatherGod]

I labelled this a WIP because I think there are some lingering questions on behavior. Mainly, as I noted in the unit tests, I wasn't 100% sure what the result should be for listreference.

[WeatherGod]

Failures appear to be unrelated.

[WeatherGod]

I am now trying to apply this feature to my needs for my work project, and I am not quite happy with the behavior of lists, particularly string lists. Yes, there is the nice symmetry that the value you get in the interpolation would be the same as what would get output to a config file, so it is conceptually compact. But, what if you need the strings quoted? What if you don't want that trailing comma?

The Template and ConfigParser interpolators are somewhat limited in what they can do. Any chance that a {}-style interpolator would get accepted? If so, we could devise a format spec for list objects. I have actually done something like that in another work project that worked pretty well (I based the format spec off of some csh variable modifiers).