Invert `xjstOptions` `flush`

[indutny]

def() should return result of other apply, without any changes or prefixing with other string.

[indutny]

cc @veged

[arikon]

@indutny What is this task about? Why is it needed?

[indutny]

@arikon right now blocks with custom renderer (in other words def() predicate), will turn off flushing behavior for all child blocks. xjstOptions is used to re-enable flushing explicitly on some templates with def(), assuming that people know what they are doing.

@veged suggested to flush all templates by default, and allow people to disable this behavior using xjstOptions. Thereby inverting current API.

[miripiruni]

@tadatuta @zxqfox @veged can anybody explain to me why we still need flush option?

P.S. Issue resolved.