Some code refactoring: sections, utils (common functions).

Code is currently broken:
    * Template is not produced by environment.
    * Template file given by argument parser is just file path, instead of
      file object.
  * New function: prepare environment.
  * Load sequence:
        [<template_path_dir> -> ] <current dir>
        -> <~/.templates>
        -> </etc/templates>
  * Get template from environment
  * Syntax fixes
Merge branch 'explicit_env_definition' into resolve_filter
New custom filter: resolve.
New example: usage of 'resolve' filter.
Changes in filter default behaviour: resolve returns IPv4 and IPv6 list
by default.
Changes in example2.jinja2: more verbose 'resolve' filter functions
demonstration.
Merge branch 'explicit_env_definition' into develop
Merge branch 'resolve_filter' into develop
Example1 layout. Nothing interesting.
New example file names structure:
    <prefix>[-<number>]-<name>.jinja2

Currently known prefixes:
    common - all common universal-configenerator examples. All examples
             with this tag should be sequentally numerated. It's like a
             tutorial.
    filter - filter usage examples.

For example:
    common-01-beginner.jinja2
    filter-resolve.jinja2
Merge branch 'examples_reorg' into develop
More verbose demonstration of '--no-trim' option.
Merge branch 'examples_reorg' into develop