Add option to explicit export method

[Alexej Yaroshevich]

Some projects using YM with global BEMHTML. Need to export in both ways.

Like that: https://github.com/enb/enb-bem-i18n/blob/master/techs/i18n.js#L17-L18

cc @artra @mishaberezin

[Vladimir Grinenko]

@birhoff, looks like that's exactly what we need for specs?

[Alexey Artemev]

Will u port changes to 4^ enb-bemxjst?

[Alexej Yaroshevich]

@artra Guess, yes.