This commit introduces hidden aliases. These are similar to hidden
indices, in that they are not visible by default, unless explicitly
specified by name or by indicating that hidden indices/aliases are
desired.

The new alias property, `is_hidden` is implemented similarly to
`is_write_index`, except that it must be consistent across all indices
with a given alias - that is, all indices with a given alias must
specify the alias as either hidden, or all specify it as non-hidden,
either explicitly or by omitting the `is_hidden` property.