Specify the set of variables to be defined in @ismatch

[gafter]

As suggested in #96 (comment), support some syntax like @ismatch (vars=A,B,C) [10,9,8] ( ... to list the variables to be assigned at the start of the block e.g. such that unmatched or unused variables take the value missing in the following:

@ismatch (vars=A,B,C) [10,9,8]  (

    [A && 10|11|12 ,    B,          C        ]  ||
    [A && 20|21 ,       B,          C        ]  ||
    [A && 1|2,          B && 2|3 ]              # note C not defined in this alternative
)