Additional stopwords

https://github.com/citation-style-language/documentation/blob/master/specification.txt says that words like 'a', 'and', 'the', 'nor', etc. should be stopwords for title case capitalization. Some of those given in the spec were missing from citeproc-ruby.

This was causing invalid capitalization for styles (like MLA) that use title case, e.g. _Declaration on Democracy, Political, Economic And Corporate Governance._ Clearly 'and' should not be capitalized here.

I have added them. I also changed to the terser %w array init syntax.

Thanks for considering!