The general idea here is to permit future syntax extensions;
as a rule, new syntax can only be added by using one of these
characters that can't be a string.
Backtick ` in particular is in the current ECMAScript drafts
for multiline strings, so supporting multiline strings with
that would be a very natural thing to do.
The other characters added here (^, !, ?, @, *, &) are
more or less arbitrary. &, ! and * have special meaning
in YAML so might be natural ones to use for similar
purposes. ? would have some natural "let this be missing"
meaning, @ is often used in template-like things, and ^
could be used for some unforeseen need.
The spec described parsing this as a 1-element path but
the implementation parsed it as 2 elements which is probably
a better behavior because it's less surprising. The simple
rule is now "only quoted periods are special".
The AbstractConfigObject.merge() method was wrong; it merged in the
wrong order, which produced incorrect results. But there was a test
verifying that the results were wrong! So it's OK right?
Anyway, fix the test, fix the code, be sure it's crystal clear in spec.
