Add support of the `escape_code` config option.

[artlomov]

I’ve made two preprocessors that work in pair. They deprecate _unescape preprocessor. To use them, special flag must be set in config. Othervise, Foliant will work as earlier, in backward compatibility mode.

It would be right to maintain them as a part of Foliant core, but I develop them as a separate package for faster updates. In the same time, I’ve needed to integrate Foliant core and Includes with these preprocessors.

Details and examples you may see in the project’s README: https://github.com/foliant-docs/foliantcontrib.escapecode/blob/develop/README.md.

The work of the entire ecosystem including Foliant, Includes, EscapeCode/UnescapeCode, MultiProject extension has been comprehensively checked locally.

[codecov[bot]]

Codecov Report

Merging #50 into develop will increase coverage by 0.02%. The diff coverage is 83.33%.

@@           Coverage Diff            @@
##           develop   #50      +/-   ##
========================================
+ Coverage    77.97%   78%   +0.02%     
========================================
  Files           13    13              
  Lines          345   350       +5     
========================================
+ Hits           269   273       +4     
- Misses          76    77       +1

Impacted Files	Coverage Δ
foliant/config/base.py	100% <100%> (ø)	:arrow_up:
foliant/backends/base.py	80.76% <75%> (-0.87%)	:arrow_down:

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