Remove the additionalHeaderContent ReSpec setting

It's not used by ReSpec, at least not any longer.
Tell ReSpec to not add RDFa

It's distracting and hardly useful.
Update CSS Selector Spec ref.

As requested by fantasai.
https://www.w3.org/Bugs/Public/show_bug.cgi?id=23788
Be consistent with the default values.

This NOTE was inconsistent.
Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=24315
Merge pull request #41 from silviapfeiffer/CSS_Selector_Ref

Update CSS Selector Spec ref.
Merge pull request #42 from silviapfeiffer/default_line_align

Be consistent with the default values.
Use a more explicit CSS selector example.

Since WebVTT is based on SRT and SRT has numbers as identifiers for
cues, the selection of cues by number is a common use case and worth an
example.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=19632
Merge pull request #40 from foolip/no-rdfa

Tell ReSpec to not add RDFa
Merge pull request #43 from silviapfeiffer/css_example

Use a more explicit CSS selector example.