Improving checking of country names.

The country name in the YAML header is used to match the filename of a flag icon.
It must be properly capitalized and use hyphens instead of spaces --- abbreviations
like "USA" and spaced names like "United Kingdom" don't work.  Rather than trying
to encode those rules, this change relies on a table of acceptable names.
Merge pull request #80 from gvwilson/checking-countries

Improving checking of country names.
Merge branch 'gh-pages' into fixing-internal-links
Addressing feedback from @dlebauer