Kast in k

I created this diff for use in managing the discussion on the KORE syntax. We may choose to close it rather than merge it, I just need a place to put comments.
@dwightguth that us not captured in this syntax, but it was discussed extensively in meetings and on the K wiki of KORE: we will use the markdown approach for nesting: ``` `` `foo` bar `baz` `` abc `` `foo` bar `baz` `` ``` is the same as `((foo) bar (baz) ) abc ((foo) bar (baz))`.  But such cases of nested K brackets are extremely rare, so it is OK for people to write a bit more when the case arises. 

So the correct syntax here should be something like this:

    syntax K         ::= "```" K "```"  [bracket, prefer]
                             | "``" K "``"     [bracket, prefer]
                             |  "`" K "`"      [ bracket ]