Support for merging multiple config files from variable

[mumoshu]

To better support the use-case of dynamically generating configs from other configs and variables, the following changes are made:

• Added paths to file config source. Either the existing path or paths is required.
  • All the files listed in paths are merged using mergo with mergo.WithOverride and mergo.WithOverwriteWithEmptyValue
• config source's paths and path can now refer to vars. variant internally builds a DAG of vars, confs, and secs to make it possible.
• Added function block to define user-functions, with support for recursive call. Note that user-functions are visible within a variant command that defines it.
  • This means two things:
    • You can't import user-functions
    • User-functions defined in the parent variant command are not visible to imported variant commands.
• Added new example for config-depends-on-var use-case at examples/advanced/dynamic-config-inheritance
• Added new example for importing variant command with local user-function at examples/advanced/userfunc-local-scope

Resolves #42