Implement initialization of configurations

[Waitak]

Viper is great for flexibly handling configurations that are written by some outside means, but it does not provide a means of starting from scratch, nor does it provide the related ability to update a configuration from Go. It would be great to have an API method that, after constructing a configuration programatically, can write the configuration to a file in the desired format. Some issues to overcome:

• If values were read in from several layers of configuration files (remote, /etc/..., ~/.config/..., ./...") then it's not obvious how to limit the values written to the file to only the ones explicitly set.
• It would probably also be necessary to note if any values have been changed since they were read. GIven the hierarchical nature of configuration sources, this is also not obvious.

Still, it seems like a generally sufficiently useful function to be worth considering.

[doublerebel]

This looks like a dup of #145, also #153 would cover exporting the config.