[custom-elements] Prefix x- for custom elements like data- attributes

[Bang Seongbeom]

It would be good to restrict custom element's name to start with like 
'x-' for the future standards. User-defined custom attributes; data 
attributes are also restricted its name to start with 'data-' so we can 
define easily new standard attribute names ('aria-*' or everything 
except for 'data-*'.)