Allow external FuncMaps

[angrycub]

Since Nomad uses CT as its underlying template engine, it would be nice to be able to pass functions from Nomad itself into the template configurations that it generates. I thought exposing a route to inject functions for applications that use CT internally might be a good middle ground to prevent even more template application sprawl.

This would allow Nomad to inject purpose-built templating functions into the rendering pipeline without having to make additional changes to consul-template.

Custom Functions could also be used to simplify the FunctionBlockList by allowing someone to implement their own no-op function to opaque an implementation provided in CT.

This could let us resolve #1312 and #1406 by allowing them to be passed into CT from Nomad using this scheme.

[eikenb]

Hey @angrycub,

We're already working on this feature as part of hashicat, the in-progress library which replaces consul-template (as a library). It's progress was paused for a bit due to CTS deadlines around hashiconf, but I'm working on it again now.

[eikenb]

We finally have CTS done enough that I've been fully freed up to get back to work on other projects and am reviewing these PRs again. I'd like to just reject/close this one, looking to moving Nomad hashicat for the feature.

@angrycub, is that OK?

[angrycub]

Sure. No reason to keep this one.