Add Fuzz.charRange

API:
```elm
Fuzz.charRange : Char -> Char -> Fuzzer Char
```

Usage:
```elm
Fuzz.charRange 'a' 'z' : Fuzzer Char
Fuzz.charRange 'A' 'Z' : Fuzzer Char
Fuzz.charRange '0' '9' : Fuzzer Char
```

Perhaps some pre-baked fuzzers could also be added (naming would need some work):
```elm
Fuzz.numChar : Fuzzer Char
Fuzz.alphaNumChar : Fuzzer Char
Fuzz.alphaChar : Fuzzer Char
Fuzz.lowercaseAlphaChar : Fuzzer Char
Fuzz.uppercaseAlphaChar : Fuzzer Char
```
as we already have `Fuzz.asciiChar` etc.!