feat: support multipart/multidoc JSON/YAML data

[tnyeanderson]

Adds the -m flag which executes the template for each line of JSON data it receives. Each line should be a complete JSON document.

It also adds multidoc support for YAML (separated by ---) without the need for an additional flag.

For both YAML and JSON multidocs, the program will error out if an error occurs by default. If the -i flag is set, the error will be printed but subsequent documents will be processed.

This also includes some new reader/writer based structs and methods to the public timplit API, and changes the Execute* functions to return []byte instead of string.