With a "+" or "-" prefix, the "metadata_to_use" setting can manipulate
the existing default.  This way, one can enable `TAG_COMMENT` without
having to list all the other tags.

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

For Android pre-5.0 compatibility (#213).

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Was broken by commit 8a86460b

Closes #87

We will reuse these tags elsewhere.