Add a hex wrapper

[scr-oath]

@dborovcanin - this adds hex support #69

1. Sorted plugin Loaders
2. Implemented hex with unit tests/readme

I did NOT make the change (to crypto aes stuff) here, but I also recommend updating your code in #69 maybe with _hex suffixes if you find them useful, but mainly to just convert strings to []byte directly.

[scr-oath]

@dborovcanin - see #71 for how the non-hex could be added. Note - I totally agree that, for testing, hex can be useful… I guess I could code up the actual use case, if slightly contrived, which is to read from file.

[scr-oath]

@vadv - This PR is discrete - while motivated by #69 - this only adds the hex wrapper, which others might find useful on its own.