Organize README and add doctoc table of contents
Move table of contents below overview
Add paranoia about ClassLoader in ConfigParseOptions

Fixes #155. Really this is kind of a mess; having overloads
with both ClassLoader and ConfigParseOptions parameters, when
the parse options may include a class loader, confuses the
heck out of things.

Trying to make it so that the ConfigParseOptions variants are
the canonical version, and the ones with a loader just immediately
set the loader on the parse options and call the variant with no
loader parameter.

Anyway ... when it fails this patch should at least make it much
clearer what went wrong.
Handle non-String values in a Properties object

The java docs for Properties say avoid this, but
it is possible, so just silently skip any non-strings
we find in the Properties.

Fixes #147
Shrink previous patch to skip non-string Properties

There was some extra/leftover logic in there. With this commit
the diff vs. the original codebase is smaller and clearer.
add 1.2.1 to NEWS
Bump README to 1.2.1
More README improvements
Fix markdown syntax in README
Settings pattern is not a way to get an optional setting
If all concatenation elements are undefined, undefine the field

This is a spec change but the spec was wrong and the implementation
previously just threw an exception.

The spec claimed that ${?foo}${?bar} would become empty string,
but it's more useful for it to just become undefined.
You can add an explicit empty string with "" if you want it
to be empty string.

Fixes #161.