support for sequences

[andreasbaumann]

rules of the form get hard to read:

Name = sequence_imm( name1 = WORD );
Name = sequence_imm( name1 = WORD, name2 = WORD );
Name = sequence_imm( name1 = WORD, name2 = WORD, name3 = WORD );
...

actually what would be nice is:

Name = sequence_imm( names[] = WORD{1,3} );

match one to 3 WORDs and accumulate the attributes as an array of strings into the attribute 'names'.

[patrickfrey]

See #41