attributes: spec out and implement

[gullerya]

So far data-tier has no special treatment for the attributes tying.
Recently, thanks to @piotrpazola , there was a functionality added, that allows to tie to the data- attributes, but this is rather a particular case, and yet the attributes tying should be handled as a whole feature.

Right now, the most relevant candidate is to express the attributes tying via the special 'directing' syntax of tying declaration:
<span data-tie="tieKey =a> hidden">... (see that a within the arrow?)...
The syntax self is not finalized and as part of it there should be addressed an issue of 'boolean' attributes, those that are either present of not, as function of:

• being null / undefined / something
• being false / true

[gullerya]

The syntax will be as the current working proposal as in the docs/api-tying-declaration.md.