Load templates from S3

**Is your feature request related to a problem? Please describe.**
As far as I understand there are two ways of getting templates works:
- put templates at the same location near executable as they were near main.go
- pack all templates into an executable with go-bindata

**Describe the solution you'd like**
I would like to use S3 storage for templates

**Describe alternatives you've considered**
It seems that locales are able to load from function, e.g. `LoadAssets`, but not a templates

**Additional context**
I don't think it's a good idea to have a big executable with all of my HTML templates (email templates - there are a lot)
Also, don't like the idea to carry along the HTML directory.