Mini notation: add underscore elongation

[eilseq]

This pull request add the underscore elongation mini notation to the related package. Expected behaviour monitored by tests is:

assert.deepStrictEqual(minS('a@2 b@3'), ['a: 0 - 2/5', 'b: 2/5 - 1']);  // @ elongation
assert.deepStrictEqual(minS('a _ b _ _'), ['a: 0 - 2/5', 'b: 2/5 - 1']);   // _ elongation