Implement # for ABNF

[katef]

RFC7230 defines the extension # which provides lists:

     1#element => element *( OWS "," OWS element )
   and:
     #element => [ 1#element ]
   and for n >= 1 and m > 1:
     <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )
...

     #element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
     1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )

https://tools.ietf.org/html/rfc7230#section-7