implement config variables prototype

[blake-mealey]

Implements a prototype of #147:

• Config files are split into separate YAML documents within the same file, separated by ---:
  • The first document is the "header" which contains the environments and state properties
  • The second document is the "body" which contains everything else
• When the config file is loaded, the header document is parsed as normal and the environment determined as before. Then, the body document is rendered as a template using various contexts before being parsed as before.
  • Templates use the tinytemplate crate for rendering. Syntax is very basic: {context.variable}.
  • Contexts:
    • env:
      • Contains the label variable which is set to the selected environment's label
    • vars:
      • Contains variables configured under environments.variables for the selected environment
• environments.*.target_name_prefix is removed
• environments.*.target_access is removed
• environments.*.target_overrides is removed
• environments.*.variables is added and will be used to populate the vars context

I do not intend to use tinytemplate for production since it is too basic, but it was useful for getting a prototype working. For a proper implementation I would like the template rendering to:

• Use ${{}} syntax so that the templates don't break YAML syntax
• Support rendering object and array values (currently it only supports primitive values)
• Support simple function expressions (e.g. coalesce, concat)
• Support simple arithmetic expressions (e.g. +, -)
• Support simple condition expressions (e.g. if <cond> {} else {})
• Support literal expressions (e.g. 'literal string')

See light/mantle.yml in this prototype for example configuration file.

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