Form: add possibility to extend form with custom elements

[sankouski-dzmitry]

There can be custom html elements in html form in some applications (for example, custom autocomplete fields).

User is able fill forms with custom html elements, by extending Form class and providing operations for custom elements in constructor.

[artkoshelev]

Please provide tests with usage example.

[sankouski-dzmitry]

@AlexanderTolmachev @eroshenkoam guys, please have a look