Separate parsing and processing of options.

Rename Options::Parser to Options::HelpfulParser - this is the class
that works like Ruby built-in OptionParser, but with improved --help
system.

Use Options::Parser as a name for the class that constructs the
OptionParser object and performs parsing with it, returning
Options::Record object.  While doing so, it tries to do little more than
setting of attributes on Options::Record - it handles converting
filenames to canonical form; reading file contents for --head-html,
--body-html and --css; reading and interpreting the --config JSON
file (and also auto-detection of the latter).

Options::Processor takes care of the rest: expanding filenames,
rendering the footer, configuring various classes, and validating
the options.