chapter generation from custom input directory

To ease translators' work, the input directory for
build_chapters.pl can be optionally provided on the command line.
It falls back to 'sections', i.e. the previous directory were the
English source lives.

This allows calling build_chapters.pl like this:

   perl build_chapters.pl translations/italian
missed >
Fixed typo
Link must be lowercase
add meta encoding utf-8 into generated html
parenthesis back
Merge pull request #2 from vlet/russian_translation

Russian translation