Convert AddConvert to store funcs in map

[patcon]

Addresses #12

[patcon]

Using numeric sort key so we know that the order of execution is the same, and allow people to choose where to insert their processing. Untested for now, but I'll start using it on my production app, and will confirm working

[lunny]

This will break the usage of the function.

[patcon]

Thanks for looking at this PR lunny! You're right, it's a breaking change and major version-bump as-is. Do you have a rough idea how you'd like to see backward compat preserved? (i'm a bit unsure what the norms are in golang)

If you have no immediate thoughts, that's ok too. Thanks!