[feature request] additional YAML helper functions

[akamensky]

Consul Template version

v0.35.0
actually used from Nomad template stanza

Request

consul-template and places where it is used (like Nomad template stanza) have methods like parseYAML, toYAML etc. However a few are missing that could make this much more useful:

• mergeYAML -- merge 2 YAML objects allowing override leaf nodes of one with values of another. mergeMap[WithOverride] does not work here as YAML gets map[inertface{}]interface{}, but explode gets map[string]interface{}.
• treeYAML -- read KV tree as YAML object, where standard YAML type inference is used. tree can be read, but it assumes everything is a string, for example with KV tree like config/app/key = 21 reading it as {{ tree "config/app" | toYAML }} will yield key: "21", which obviously is not what would be expected.

[akamensky]

added mergeYAML in #1846

[akamensky]

added treeYAML method in #1847