Function filter

pub fn filter(ps: &mut Vec<Problem>, query: String)

Abstract query filter

    -q, --query <query>          Filter questions by conditions:
                                 Uppercase means negative
                                 e = easy     E = m+h
                                 m = medium   M = e+h
                                 h = hard     H = e+m
                                 d = done     D = not done
                                 l = locked   L = not locked
                                 s = starred  S = not starred