Refactor MassEffect2.in adapter and improve it.

Refactoring:
    * New `Chapter' class extracted to separate HTML parsing from
      adapter output preparation and story-wide metadata collection.
    * Lazy-parsing and fragments caching is done at Chapter level.

Improvements:
    * Rating "adultness" and label-to-title mapping made configurable.
    * Fix chapter number detection when title contains large numbers
      (ex.: http://www.masseffect2.in/publ/19-1-0-2934).
    * Add mechanism for detecting series-like stories with no chapter
      numbering and extracting correct titles.
    * Fix number format for generated chapter titles.

calibre-plugin/plugin-defaults.ini

@@ -1230,8 +1230,11 @@ extracategories: Mass Effect
 ## May be adjusted in `personal.ini' on per-story basis.
 never_make_cover: true
 
-my_custom_label: Some text
-my_custom_setting: true
+## Titles for ratings identified by 1- or 2-letter codes from `ERATING system'
+## (`система Р.Е.Й.Т.И.Н.Г.').  MassEffect2.in and some other sites adopted it,
+## but changed titles and update them occasionally.
+rating_titles: R=RESTRICTED (16+), E=EXEMPT (18+), I=ART HOUSE, T=To every, A=IN=Иной мир, Nn=Новый мир, G=О\, Господи!
+adult_ratings: E,R
 
 [merlinfic.dtwins.co.uk]
 ## Some sites require login (or login for some rated stories) The

fanficfare/configurable.py

@@ -205,6 +205,7 @@ def get_valid_keywords():
     return list(['(in|ex)clude_metadata_(pre|post)',
                  'add_chapter_numbers',
                  'add_genre_when_multi_category',
+                 'adult_ratings',
                  'allow_unsafe_filename',
                  'always_overwrite',
                  'anthology_tags',
@@ -285,6 +286,7 @@ def get_valid_keywords():
                  'output_filename_safepattern',
                  'password',
                  'post_process_cmd',
+                 'rating_titles',
                  'remove_transparency',
                  'replace_br_with_p',
                  'replace_hr',

fanficfare/defaults.ini

@@ -1825,14 +1825,17 @@ extracategories:Lord of the Rings
 
 [www.masseffect2.in]
 ## Site dedicated to this fandom.
-extracategories:Mass Effect
+extracategories: Mass Effect
 
 ## Stories on the site almost never have cover image.
 ## May be adjusted in `personal.ini' on per-story basis.
 never_make_cover: true
 
-my_custom_label:Some text
-my_custom_setting:true
+## Titles for ratings identified by 1- or 2-letter codes from `ERATING system'
+## (`система Р.Е.Й.Т.И.Н.Г.').  MassEffect2.in and some other sites adopted it,
+## but changed titles and update them occasionally.
+rating_titles: R=RESTRICTED (16+), E=EXEMPT (18+), I=ART HOUSE, T=To every, A=IN=Иной мир, Nn=Новый мир, G=О\, Господи!
+adult_ratings: E,R
 
 [www.mediaminer.org]