Consider allowing Sprig template functions for the Profile Registry feature

[thefire]

Hello,

I just found and read #856 while looking at whether Granted's profile registry feature could use any go template functions. This would be super handy in this feature where you want to build up reusable aws profile snippets.

Would you consider a PR to actually introduce Sprig template functions into the awsmerge package? I would probably also change it from using html/template to text/template since that makes more sense.

Specifically we want more power to be able to deal with "prefixed" profile names. Just using .Profile is not enough cause we also have role profiles that need to specify a source_profile which can then be prefixed sometimes.