Remove duplicated display() class method from RendererGroup, fixing bugs

This is actually achieved by replacing the version in RenderGroup's parent
class AnalyzerRenderer (which was buggier, missing one esc_attr() and an
"\n" in bare mode) with the one from RendererGroup, and replacing one
is_null() check for the contents of a member variable by an isset() (which
extends the check to the existence of that member).
Merge pull request #208 from ockham/remove-duplicate-display-class-method

Remove duplicated display() class method from RendererGroup
In the PHPdoc for scan_theme, change the scan type default value note to correspond with the actual one
Allow usage of default arguments with analyze-theme CLI subcommand