bemhtml: fix initialization of global properties

They should not be initialized to `context[prop]` by default.
bemhtml: fix globalizing variables in hashmap
bemhtml: introduce tree structure checks
Store block params in data attributes
i-bem__dom: Fix for blocks without domElem. Closes #460
i-bem: propagate mods and elemMods to mix mode tpl
Merge pull request #506 from bem/test@2.x

Bulk PRs merge

i-bem__dom: Store block params in data attributes
i-bem__dom: Fix for blocks without domElem
BEMHTML improvements