Add new pragma alternative

[awalterschulze]

%% @format seems to conflict with edoc
We need to add an alternative pragma and support both for backwards compatibility
We also need an option to update all %% @format pragmas to the new pragma
I guess this can just be using --insert-pragma on a file that already has an old pragma

[awalterschulze]

See conversation in AdRoll/rebar3_format#243