Support SQL comments

[elliotchance]

<comment> ::= <simple comment>
  | <bracketed comment>

<simple comment> ::=
  <simple comment introducer> [ <comment character>... ] <newline>

<simple comment introducer> ::=
  <minus sign> <minus sign>

<bracketed comment> ::=
  <bracketed comment introducer>
      <bracketed comment contents>
      <bracketed comment terminator>

<bracketed comment introducer> ::=
  /*

<bracketed comment terminator> ::=
  */

<bracketed comment contents> ::=
[ { <comment character> | <separator> }... ] !! See the Syntax Rules.