Although many of the examples must be run from the example directory as
cwd, everyone getting started probably wants to try the examples, cmake
knows how to do it, so let's enable it by default.
This adds an optional feature LEJP_FLAG_FEAT_OBJECT_INDEXES that changes
lejp to treat { } items as indexable in ctx->i[] / ctx->ipos, since they
also can take commas.
This may break existing uses so it requires the default-off feature flag to
enable it. The flags context field is zeroed by lejp_construct(), so any
flags should be set on ctx->flags after alling that.
There's also a flag LEJP_FLAG_LATEST available as an alias to enable any
desirable but not-backwards-compatible behaviour, including this.
Add the info to the README and adapt the unit test to do it both with and
without the FEAT_OBJECT_INDEXES flag.
