Refactored to use templates and allows vectors to be read from settings into a std::vector container

This pull request involves refactoring the code to use templates & specialisation, instead of overloads, which reduces code duplication, and allows easier changes to be made.

The switch to a template based system was chosen to allow reading of comma separated strings into a vector (which type is determined by the user). It also allows writing of vectors to the file.

(Now with only one commit!) :)
Well if it is changed, I feel like that'll result in inconsistent behaviour, as primitives allow the same behaviour.

To fix that, I feel like the get functions should return the type, instead of changing the passed in value. 
This is pretty simple to do, replacing it with `get<T>(const std::string& key)`