Support `--characters` from a file

I want to test the following chars:

```
!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ ¡¢£¥§¨©ª«®¯°²³´¶·¸¹º»¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿĀāĂ㥹ĆćĊċČčĎďĐđĒēĖėĘęĚěĞğĠġĢģĦħĪīĮįİıIJijĶķĹĺĻļĽľĿŁłŃńŅņŇňŊŋŌōŐőŒœŔŕŖŗŘřŚśŞşŠšŢţŤťŪūŬŭŮůŰűŲųŴŵŶŷŸŹźŻżŽžǍȘșȚțȷˆˇ˘˙˚˛˜˝ẀẁẂẃẄẅẞỲỳ –—‘’‚“”„•…‹›⁄⁰⁴⁵⁶⁷⁸⁹₀₁₂₃₄₅₆₇₈₉€™−◌
```

I can't though, because there's special characters in there, so I have to fight my shell's escaping rules and then ninja's (because the string is inserted verbatim). Please add a way to have the code read the characters from a file.