Allow single-line comments

[nwagner84]

Examples

$ cat filter.txt
041P{
    a == 'Algebra' && // Sachbegriff
    2 == 'lcsh'       // Code der Quelle
}

$ pica filter -f filter.txt "[email protected]?" tests/snapshot/data/algebra.dat | pica count --records
1

$ cat filter.txt
041P.a == 'Algebra' ||  // Sachbegriff
041P.2 == 'lcsh'       // Code der Quelle

$ pica filter -f filter.txt "[email protected]?" tests/snapshot/data/algebra.dat | pica count --records
1

Closes #457