Make it easier to swap out the loader, to allow for a custom loader

[nathanmac]

Small PR which just moves the loader to a place where it can be easily overwritten to allow for custom loaders and adds a LoadedInterface

My particular use-case for this concerns some additional pre-processing steps necessary to allow the .env file to be readable, therefore requiring a custom loader with a minor addition to the existing loader, this would make it easier/cleaner to overwrite the loader in the Dotenv class.

[danhunsaker]

Doesn't look like this change actually lets you supply your own loader instance at any point... It just adjusts the abstraction a bit to call a method rather than directly construct anything inline.

[nathanmac]

That's correct the idea was to remove the double instantiation so the get loader method could be overwritten in a single place, this appears to have been addressed in another PR #215.

[GrahamCampbell]

@nathanmac NB Further improvements have been made in 3.x series. :)