Consider breaking the gomplate cmd into a sub-package

[hairyhenderson]

As suggested by @sean- originally in #145 (comment):

Have you thought about breaking gomplate into a sub-package like github.com/hairyhenderson/gomplate/cmd/gomplate so that the "gomplate" template library could be the meta-library used to aggregate a bunch of different APIs and helper functions? It's pretty easy for me to see that this would be useful to embed in standalone programs. "Dear gomplate, please render my string." Either gomplate(1) or the library github.com/hairyhenderson/gomplate.

🤔

[nickjackson]

Yeah, this would be really helpful. I'm looking to create a cmd line scaffolding tool for my team, and leveraging gomplate would take a lot of the heavy lifting away.