config: Replace Config._setup() with Config.setup_html2text()

Since 7bbbf62 (Setup html2text in Config._setup(), 2012-10-04), the
html2text configuration options have only been referenced from
Config._setup(), and that method was never called.  With this commit,
we rename the method to setup_html2text(), and add a new
Feed._html2text() which invokes the setup before calling
html2text.html2text().

This caused a fair amount of churn in the expected test results, as
previously ignored default values for html2text kicked in.  I also
added a test exercising the non-default values (allthingsrss/3), and
it looks like everything works as expected.

Signed-off-by: W. Trevor King <wking@tremily.us>