Fix for page.GetParam() for JSON and TOML maps

Fix for page.GetParam() for JSON and TOML maps

Setting per-page Blackfriday angledQuotes did not work
with TOML or JSON front matter, but it does work with YAML.

It turns out that page.Params("blackfriday") returns
type map[interface{}]interface{} for YAML, but
type map[string]interface{} for JSON and TOML.

This patch updates page.GetParam() to catch the latter,
with an error message if page.GetParam() does not recognize
a type.  A test is also added.