Sync Options::Record field names with option names.

So that the option name used inside JSDuck is the same as the --option
used on command line.

Rename the following:

* categories_path --> categories
* output_dir --> output
* template_dir --> template
* img_tpl --> img
* link_tpl --> link
* imports -->  import
No more access options object as Hash.

Always expect it to be an object with accessor methods.
As a default (and in tests) use OpenStruct.
Fix regression in validation of options.

Was broken since we implemented names for validators.
Eliminate :header attribute from options.

Only leave the :title and calculate later the header from it in
Web::IndexHtml class.
Refactor options tests.

Avoid repeating option names as :foo and --foo.  Instead use a
function to transform from :foo to --foo.
Don't scan for CSS files.

JSDuck is really only meant for documenting JS and SCSS files.

Fixes #474