work through @dfilatov comments on i-bem.js docs
work through @dfilatov comments on i-bem.js docs

    - rewrite section on modifiers (beforeSetMod)
    - add note on static binding of BEM events
    - smaller fixes in text
    - highlight FIXMEs and add some new ones

jifeon/bem-core,robot-xliff-farmer/bem-core,tazminweb/bem-core,robot-xliff-farmer/bem-core,robot-xliff-farmer/bem-core,megatolya/bem-core,gela-d/bem-core,gela-d/bem-core,jifeon/bem-core,jifeon/bem-core,megatolya/bem-core,tazminweb/bem-core,megatolya/bem-core,tazminweb/bem-core