Add init extra that installs foliantcontrib.init.

[moigagoo]

With this extra, we can simplify Foliant installation instructions from pip install foliant foliantcontrib.init to pip install foliant[init].

[codecov]

Codecov Report

Merging #51 into develop will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           develop   #51   +/-   ##
=====================================
  Coverage       78%   78%           
=====================================
  Files           13    13           
  Lines          350   350           
=====================================
  Hits           273   273           
  Misses          77    77

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[moigagoo]

Changed that from init to mkdocs to cover the most common case.